We have had a breakthrough in training our Bulldog, we've figured out his aggression trigger. Now how to stop?

I've posted several questions these last two days about dog aggression and resource guarding ect...Today we had a breakthrough in determining his biggest trigger of dog to dog aggression..We were practicing commands like sit/stay with one of his favorite toys (black kong) He gets super excited and it's a great motivator for training. One of the biggest issues we've been having is that if we're at the park or with other dogs if they come up to sniff his , or get near his hind end he freaks out and a brawl starts. Well today i was giving commands like sit/stay and my husband sat behind him and when he would lunge or jump for the toy id turn around back to him for a few minutes and my husband would grab his collar and place him back were we wanted him. Well my husband discovered if he reached out and touched his hips or back and while he waited for the toy he became mean and snapped or barked at my husband. He didnt even have to be holding the hips just placing a hand on him near the ock area set him off. How can we desensitize him from this behavior? We did notice this all seems to have started occurring after a vet visit in which a temp was taken there. He didnt like it and has been off/on aggressive around his ever since.. What are some tips or ways we can get him to not go nuts on anyone who gets near his behind? This was the first time he's gotten upset when a human touched his back end while another one was doing something with him.Usually it's just dogs, Because, dogs are the only ones around his hind end while the people have a tennis ball, or toy..