I had a male boxer that I had from a pup. I took him to puppy socialisation classes and he was great. Unfortunately he was attacked by a rottweiler at the dog beach. He became a bit dodgy around other dogs but we continued training classes, socialising etc but I didn't trust him off lead with other dogs except my female anymore. I was then at a friend's rural property with the dogs one day and a stray dog attacked both my dogs. After that he became extremely aggressive. We saw a number of different trainers and behaviourists, and even a dog psych. Nothing worked so I resigned myself to walking him very early in the morning, always on a lead and muzzled. We had a few unpleasant moments when we came across dogs off the lead whose owners couldn't control them but generally it was a workable though not ideal situation. Unfortunately he never improved and one day finally turned on my other dog, and then me when I intervened. Both me and my female dog were injured. After long chats with my vet I made the heartbreaking decision to put him down. It broke my heart that because of other people's inability to control their animals mine had been terrified as a pup, and that then shaped his behaviour.

My SIL is currently dealing with this problem with a portugese water dog she adopted as a 15 month old after falling in love with mine. Her guy and his past owner had been attacked while out walking one day, and as a result he is now very aggressive towards other dogs. The previous owners and now my SIL are trying all sorts of different methods, and SIL is devoting a heap of time but sadly nothing is working so far.

I hope the group training and a bit of exposure in such a controlled environment makes a big difference for your guy,