if he is still mounting mares, take him out of the pasture. If he does successfully mount a mare, not everything might be blank AND the bigger concern is that introducing bacteria into a mare can cause an infection that can lead to them being sterile. If that was my mare and she developed an infection and became sterile, i'd be pretty hocked off.

have you given any thought to maybe putting vapor rub on his nose? It masks the smell of mares in heat. The best solution here is to keep him separate from the mares, but maybe this could be an alternative for you.