Aggressive rescue dog
My partner and I adopted our dog last August from a dog's home who we're unable to tell is anything about his past. Buddy is very loving toward my partner and has made us very happy. However, busy has shown signs of aggression towards any other males (humans) that he encounters. One night we had my sister and her boyfriend came round for dinner, Buddy had met my sisters boyfriend before and had been fine with him, however, on this occasion after we had dinner out of nowhere Buddy snapped and bit my sister's boyfriend, breaking the skin. We went through all our options with what to do, and considered having to have him put down but decodd we wanted to give him a chance. We bought Buddy a muzzle to wear when outside and we put him in a different part of the house when any males visit. Today my partner took him to the vets for his booster injections, it was a male vet and my partner ensured Buddy had his muzzle on, Buddy growled at the vet and at one point tried to go for the vet and had to restrain hom. The vet said that there is nothing that can be done about his behaviour as he is 3 and is ingrained.
We can't run the risk that Buddy won't hurt anyone else and we don't know what to do. Any advice please?