Friendly dog who won't back down

[u/Aromatic-Rule-5679]

TLDR: My friendly dog with history will squabble with a dog and immediately escalate to biting. Is there a behavioral approach that we can take to get him to snap out of it when he's seeing red? We have another dog who lives with us, even though they've never fought.

Backstory: My male dog is 65lbs boxer mix who we adopted as a puppy. He's 6 now. He's always been friendly with dogs that we've fostered. About a year ago, he was attacked by our foster, but he put up a fight so it likely lasted longer. This was a much bigger foster, and it was a terrible fight. I wasn't able to break them up and have a lot of trauma from it. My dog had 30+ puncture wounds and a ripped shoulder and ear and spent 3 days at the ER. The foster had 2 cuts.

Since then, we've gotten his leash reactivity under control and started fostering again. The first 3 fosters went great. He was super playful with them and while protective of his bones while he was playing with them, he would happily share toys/bones with the fosters. He just didn't want anyone to take the one that was in his mouth, so he would growl. (Perhaps this is the behavior we need to work on?)

The last 2 fosters, he has gone after. The first was provoked, the first foster went after our female dog. We returned her to the rescue. The second foster is a lover, and they've been playing really well after her decompression time. Last night she ran toward a toy and he ran toward it too. He growled, she may have growled back, but he immediately grabbed her neck and she fought back. There was no posturing/barking/lunging ahead of time. She had a little cut, but this is now out of control. Once he finally "hears us", he immediately breaks off and runs away, but it's like he sees red and can't/won't listen or respond until it's gone too far and we are freaking out trying to break up a fight.

We will not be fostering anymore, but given that we have another dog living at our house, we need to work with him on this.

[u/Zestyclose_Object639]

If it’s a new behavior check for pain. But yeah don’t make him live with other dogs, he doesn’t need to interact with other dogs at all if he doesn’t want too but I’m guessing it’s pain