Sometimes an illness or injury that is often at the root of aggressive behavior. When the aggression has not continued long enough to become a habit, curing or controlling the physical cause will in some cases immediately stop the aggression. Dogs have a survival instinct to hide their pain. Showing any sign of weakness in the wild could get them killed. Aggression is often the first and sometimes the only change in behavior from pain, as the dog acts to ward off approaches and touches that the dog has learned will hurt.
When was he last checked by your veterinarian?
I can see you're joining and somehow not being able stay in the chat room. I will switch this window to a Q&A and will await for your answer.
He gets checked by the vet regularly. The incident with the child/surgery happened over two years ago. He now goes after children unprovoked when they put their hand out to pet him. Adults do the same thing and he lets them pet him
It is possible that his previous experiance associates children with pain. Have you worked with a dog therapist since?
No I have not worked with a dog therapist. My husband is in the military and we are currently stationed in Germany.
Your goal is to build in your dog a belief system that most encounters with children will be safe. Your dog learns from experiences, and those experiences need to illustrate the message you wish to teach the dog.
A dog who has high-quality positive experiences with humans may still not be adequately socialized if there are not enough experiences. Let's say you have your dog Dexter out for a walk and a passing man frightens him. Perhaps the man crashes into Dexter, drops something on him, or steps on his tail. Maybe it's accidental, maybe the man is under the influence of some substance, but either way, Dexter has a bad experience. Just like a three year tried to lay against him after surgery.
If when this happens to Dexter he has previously encountered 50 men on outings, 40 of whom ignored him and 10 who gave him treats, what is Dexter's opinion of men likely to be? "Gee, men are usually okay, but that guy was strange!" Give Dexter several good experiences with men soon after this experience and he'll likely put it into the perspective of many good experiences and decide not to worry too much about men he meets.
If Dexter has inherited a difficult temperament, he may require more good experiences and more time to offset his bad experience. The same is true if Dexter has not had a large number of good experiences before this unfortunate one.
It's even possible that Dexter will never be able to handle exposure to men, or to whatever type of person he decides to worry about. All dogs are not equal when it comes to the socialization they need and how they will be able to handle the world, with or without good experiences. All you can do is your best.
Keep in mind that aggression is not a do-it-yourself project. Ideally you should ask your veterinarian to recommend a behavior specialist in your area. But the fact that you're in Germany this maybe difficult. I'm curious if there are any US Military Veterinarian at where you're stationed. If there are any military dogs there may also be a trainer who can work closely with you.