
I would take him to classes, both socialising and obedience. He needs to learn this is not acceptable. I think you are right to keep him on a lead, imagine what might of happened if he were loose. Unfortunately, there are dogs that will not tolerate other dogs, Boris may be one of these, however there are a couple of things I would try if I were you.
1. Have a Tardak injection (chemical castration) if he is not already neutered.
2. Get him a muzzle.
He may pick up on YOUR nervousness and act defensivley towards other dogs, a muzzle should make you relax knowing he can do no damage, and then he may do too.
Tardak may produce an effect that would mean castration is a good option, if it doesn't help there are no lasting effects and it wears off within 4-6 weeks.
Dawn.