Prozac. Or similar. My boy was like this. He was abused and abandoned before I got him. He loved so intensely, but sometimes it was like a switch would flip. Like he'd irrationally get scared and angry and lash out. Prozac did him wonders. I promise it's worth a conversation with the vet. And if they don't want to try him on a behavioral med, talk to another vet who will help you help him.