sounds like dog fear aggression, its worse on leash because the dog feels like they have to act aggressive because they cant exactly run away. the best thing you can do is make she he knows that you will protect him that he does not need to defend himself, also if he behaves this way do NOT punnish him, it will reinforce the behavior instead give him a tone of praise and treats when he looks at another dog without growling or snarling, even if its only for a split second make sure you reward it.