Sorry to sound so blunt, but you need the professional help of a behaviorist (not just a dog trainer) ASAP. This is a very serious problem, especially since he's turning on you when you try to restrain him.
In the meantime, turn and walk the other way at first sight of another dog. Carry VERY high value treats (tiny pieces of hotdog, cheese, etc.) to try and distract your dog as you exit the scene. Walk at times when other dogs aren't likely to be out (super early a.m. and late p.m.). I'd also muzzle this dog when out in public. A basket type of muzzle still allows panting. You don't want to end up getting arrested for having a dangerous dog, if things go south, or getting hurt yourself.
