I totally disagree with your vet. He should have been neutered as close to 7 months as possible (sexual maturity). At 18 months, he is an adult and will feel like he should be the boss. It is at this age that many challenge their owners to see if they are more dominant. As the dominant member of the family (because they can bite), they become unmanageable. So I am not sure why your vet recommended not neutering him. It is the first thing we suggest for many behavioral issues. It doesn't also correct a behavior and it takes a month for the hormones to be reduced, but it can take away at least one reason for being dominant (the urge to mate).
Many dominant dogs are described as well behaved until you try to get them to do something they do not want to do, and then they reprimand you either with a growl or bite if you don't heed the growl. Things like taking away something they want, making them move when they don't want to, waking them up, etc can cause them to reprimand (bite) you. You see hie is reprimanding you when you don't let him do what he wants when on the leash.
Home training is great, but it usually is not structured. With home training, it is frequently when you think about it and not a daily thing. In addition, there is not finishing of the training by ensuring the dog listens despite distractions. Group training also allows for socialization and obedience around other dogs. Your dog missed out on this which may be part of the problem now. Dogs are aggressive for a few different reasons. One is fear based. They attack before the other dog, person, etc can to scare them off first. The second reason is to dominate the other dog or person. I really think your dog is dominant and that is the problem. He doesn't see you as the boss.
The following site is helpful for teaching you how to train your dog. I know you have done a lot, but this goes over positive methods that help the dog learn to obey all the time. It is treat based and I recommend vienna sausage slices or liver slivers. NOrmal treats are not tempting enough. Be sure and click on the link to the page on obedience at the bottom. and links on subsequent pages leading to detailed instructions.
Training works best if you train at least 30 minutes a day (two 15 minute sessions). I would start making your dog work via the Nothing in life is free program (NILF). It is outlined below.
Unfortunately, the site does not allow me to make clickable links, so to view the supporting websites, you will need to copy and paste the link into a new browser window or tab.
Dogs that are allowed on furniture tend to feel that since they are elevated to your level or higher if on your lap, they mentally feel elevated as well in the pack order and thus are the boss. Keeping them on the floor can help lower them mentally back to a submissive position in the pack. So the first thing is to not allow him higher that the humans or even on the same level. Attach a leash and use it to remove him from the furniture. Give a correction in the form of a quick tug and firm "NO" when he attempts to get on and a treat when he starts not trying to get on the furniture. Thus you are providing negative reinforcement for the getting on the furniture and positive reinforcement for the desired behavior (not attempting to get on the furniture). This along with regular obedience training will help you establish dominance over him. DO NOT try forcing your dog into submission. It just creates a dog that bites since the dog feels that if you use physical reprimands, then they are ok.
If you are worrying and tensing up about what will happen, then that may be contributing to the situation as well. The dog senses the owners worry and feels that he is justified in his aggressive stance because you are obviously worried about the dog. They don't know you are worried about them attacking, they just feel that you are worried and assume it is the other dog.
If he grew up with your sisters dogs then he would get along pretty well with them. He may see them as boss if they are older then him especially if they are female. If male, you may see fighting start between them, so keep monitoring them.
That type harness can be helpful in situations like this. I prefer a chain collar but I do NOT make it tighten too much. I'm going to go over leash training since a dog should be at your side and not in front of you. When in front, they are making the decisions (dominance again).
Number one, put your dog on a leash before leaving the house. Make your dog sit or lie down before leaving. You walk out first and the dog should follow you out. With a proper walk, the dog should be right at your side or slightly behind. You dog should be paying attention to you, frequently glancing at you to be sure you haven't changed your mind about where you are going. I will be using the word correction. A correction will indicate a short quick tug and release of the leash. It is meant to remind the dog that he is supposed to be paying attention to you. Initially, keep training sessions short and where there will be minimal distractions even if it is just in your yard. A walk should be fast paced and not a stop and start exercise. The dog should not be investigating, sniffing or socializing on the walk. Walk to a destination and allow the dog some time at the destination to do those things.
I use a food and praise reward system. I use almost paper thin pieces of hot dog (vienna sausages) as the oil from them coats your hand and keeps the smell on your hand. Let the dog smell the treat in your closed hand. This gives your dog motivation to be by your side. He should be happy to follow your hand around the yard. Keep your leash short, but without pressure on it. If the dog starts moving away, a correction toward you should be made. Give him a treat every once in a while initially so he understands walking by your side get him treats. Try to time it so it is before he gets distracted. If he starts to glance elsewhere, give a correction and tempt him with sight of the treat. When he is back to paying attention, reward him with the treat in a low calm "good boy". No excitement to your voice as you want him calm. Repeat when you think his attention is shifting. As he gets better at paying attention to you and your "smelly hand", make corrections giving more praise and less treats. Before you know it, your dog will be walking right next to you all the time, with or without treats. When you stop, praise your dog with your voice or a few pats to let your dog know how good he has done. You can train him to sit or lay down when you stop if you want as well. This helps prevent his trying to run off if you stop to talk to someone.
Once your dog is pretty much always walking at your side, you will want to make a correction any time he stops paying attention to you. For instance, they are looking at a cat in a yard, give a correction so they look at you. if he is busy looking ahead and hasn't glanced at you for awhile, give a correction and reverse your direction. Do not stop and wait for the dog, just a quick correction and reverse and walk. They learn to keep an eye on you as well as on what else is going on. Try an be confident during these training sessions. Try not to look down at your dog but more out of the corner of your eye. Act like you are paying attention to the scenery. It sounds strange, but it does work.
Once your dog is doing well in the yard, try adding a few distractions such as family in the yard, then progress to another dog around continuing to correct if he even looks like he is going to glance at the other dog. If you wait till he is already distracted, it is too late. You have to catch him before he focuses on the other animal or person. It is a lot of work and takes lots of practice but it does work.
His tail should be level with the body or down. If it is up, brush it down with your hand. As silly as it seems, keeping that tail down can keep the dominant behavior in check.
These things should all help you control him around other dogs and on the walk. It is going to take a lot of practice and definitely get him neutered as soon as possible. There is another training technique gaining i popularity. You can read about it here:
You may want to go ahead and get a basket muzzle for him and use it on your walks. It will prevent bites from occurring and ease your mind allowing you to be much more relaxed and less tense. The basket style also allows you to give treats, etc and lets them drink and pant normally.
I hope this information is helpful to you and you are satisfied with my response. If you would like any additional information or have more questions please don’t hesitate to press the reply to expert or continue conversation button so I can address any issues you still have .
If you have questions in the future that you wish me to answer, you may go here http://www.justanswer.com/pet/expert-jane-lefler/ and bookmark the page or make it a favorite. It is best to put my name "JANE" in the question as well. Please recommend me to your friends and family members if they have any problems with their dog as well. I would truly appreciate it.