Thanks for the additional information. It is helpful. If the vet has not performed a thyroid panel, you should have that done. This is a condition that requires special tewting. Hypothyroidism can lead to sudden aggression in a dog and cockers are prone to it. Read about medical causes for aggression here.
It sounds like your dog may be having issues with dominance aggression. 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. When he was in pain, he may have found that acting like this got humans to stop doing things or to move away. 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 or snap at) you.
There are ways to regain the dominant position in the house. The best way is to start obedience training back up. While a formal training class is great, you can start obedience training without a formal class. Before you can get into classes, I am including links to a couple of other sites that teach some good methods of training. Be sure and read both.
The following site is also helpful. 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.
I think you will be pleasantly surprised at how well your dog does with additional training if no medical conditions is present. Dogs love the little paper thin slices of hotdogs that I use for treats while training. Give this a try and see how it works for you.
Additionally, I would suggest you get a basket muzzle and make him wear it anytime children are around or visitors are there. Be sure to use the leash to make him obey you. If he growls give a short tug to get his attention and a firm "NO" to let him know, you are not going to allow his aggression. If he is sleeping, give a little tug to let him know that someone is there so he isn't startled when being woken up. .
In addition, if the situation is not improving using the techniques on the previous website, you may have to consult a professional behaviorist. You can usually find a behaviorist by asking your Vet for a recommendation or you may be able to find one using the following site.
I would not change the food unless your vet wants him on a special diet. If he is overweight, I would get him back to his suggested weight by cutting his portion in half and increasing exercise until he is at the desired weight. Overweight dogs can be snappier due to discomfort from strain on the joints, etc. Hypothyroid dogs often are a little overweight or have that appearance.
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 .