Yeah, sounds like a fear issue to me, especially if he was present when the other dog was attacked.
I'd try redirect-and-reward with him and see if you get any results. Whenever he shows aggression (like on-leash at a dog park) turn him away and start walking the other direction. Whenever he is calm around other dogs, praise and reward him with something really tasty.
He should come around eventually. If not, I would see professional help from a local trainer. |