We did have a dog when I was growing up that would attack every other dog she saw. Raised from a pup, with our other dogs, no reason for it at all. I took her to obedience classes and she started off having to sit away from the other dogs, snapping at any that got close. I took her for a couple of years and by the end of it she was fine with other dogs. Sometimes she would even play with them. We were able to let her off lead at parks with no worries that she would attack any other dogs. So it can happen. Group classes are a great way to get him socialised.