My thought are that if there is even the slightest chance that you dog will attack another dog then you really need to keep him on a lead or muzzled. I notice that someone else has said that they would not muzzle in caae he were attacked, I don't necessisarily agree with this because at the end of the day you and you alone are responsible for the actions of you dog - if that means muzzling then sadly muzzle you should do.

Have you considered seeking the advise of a behaviourist - and is it at all possible that he can sense that you are uneasy due to this problem - he is picking up you uneasiness hence causing his agression.
All the best
regards
DT