i bet that might be the problem mate as from a young age they are learning how to interact with other dogs my advive would be bring him on a lead to where there are dogs and very slow introduce him to other dogs and when he interacts in a nice way lots of attention and sugar lumps break them in half's though and try not to use them to often if he learns he gets good attention and sugar lumps for interacting in a good way he should learn its kinda the same thing with pigeons trying to teach them good habits you can teach most dogs to do anything with sugar lumps