I know you think his behavior is not aggressive but barking at people and chasing them or running up to them and even nipping at them is indeed dominant and aggressive behavior. He is still young and often it is around 18 months of age that a young male will test their owners leadership and challenge them with a growl just to see if they can get away with it. If it isn't corrected immediately a dog will push its boundaries even more. So it is important to get this corrected as soon as possible.
I'm going to suggest some formal obedience training in a class atmosphere. this will teach him to obey despite distractions. I'm also going to give you a great site that teaches you some great methods of training. 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). It needs to be set times where this is all you are doing so he isn't distracted during this part of the training. I would start making your dog work via the Nothing in life is free program (NILF). It is outlined below.
These things will reinforce your place as the alpha/boss. I also want to be sure that you have a good recall or come with him. Many dogs don't come when called because they have learned that the only time they are called is when fun time is over. People call their dogs to them to make them come inside or to stop chasing prey (cats) or to be put on leash (end of free running time) or even crated. The only association they have with the come command is negative. Additionally, dogs find chase to be a highly amusing game and have learned that if they get close to a human, the human might chase them. They love a good game. So what you need to do is make coming to you more pleasurable.
The easiest way is to reward your dog with small tiny treats and praise whenever your dog comes to you when you give the command. Do this even when the dog wants to come to you. After a few treats, the dog will associate coming to you with getting treats and praise. Outside, you will want to use a long lead. Do not drag your dog to you, but say the command and if the dog doesn't come, give the leash a short tug. Start with short distances and gradually extend the distance as your dog becomes more familiar with the command. Over time, you will reduce the treats and increase the praise until praise is the only reward. Another thing to remember is to never call a dog to you to discipline it, go to the dog. During training I don't call a dog to me unless it is going to be pleasant for the dog. I usually don't have much of a problem since the dogs quickly learn that I have thinly sliced hot dog treats just waiting for them to obey me.
Practice several times a day calling him to you and rewarding him for it and then give a release command so he knows he can go back to running or what ever else he was doing. If you have a good recall, you can stop any behavior.
Get him a back pack and fill it with water bottles and cans to give him more weight and help tire him out more during walks. Keep a light weight lead on him so if he starts to bark or lunge at anyone, you can use the lead to get his attention by giving a short tug and a firm low toned "NO" of correction. Right now he gets a reaction from the people he chases and if they continue moving he nips at them. He needs to know that this is unacceptable behavior. To him it is game more than aggressive but is turning aggressive. People jogging and running do become scared of a large dog charging them and their fear will encourage him to be aggressive.
If working with him off leash, put a basket muzzle on him. It will allow him to pant normally, drink and eat but won't allow him to bite or nip. Practice that recall until he responds immediately every time and use that to intercept his charges. As he matures, he will be less likely to exhibit this behavior especially if you are consistent with your recalling him .
Get a very lightweight lead so he forgets he has it on and you can use that as well. I have an appointment, so if you need more direction, just respond and I'll answer when I return.
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 .