Help for my friend's rescue crossbreed?

My friend recently got a rescue dog-he's about 7 and the rescue centre believe that he is a staffordshire bull terrier/labrador/collie X He is wonderful around people-friendly and gentle, but if he meets other MALE dogs on a walk, he will growl and snap, and once got into a fight with a pitbull. We are all very worried about the other dogs, and the safety of whoever is holding his lead-he really can pull when he wants to, We, and the rescue centre, can only imagine that he was raised a 'macho' status symbol sort of dog, poor thing, but his behaviour became too much for them and so he was abandoned in the Brecon Beacons in December-he was found starved, soaked through and shivering. He has been neutered, and it has been suggested that this is why he feels vulnerable around other males. He is very territorial and protective of his family. Can you imagine anything that can be done? Thankfully they live in quite and isolated area, where it is possible to take him for a 2 hour walk without meeting any other dogs at all, but it would be wonderful if they could take him out with them. Thanks! I did try to keep it concise but there was so much to say!