I really think you should try to seperate him from the mares that won't kick him. Either put him with the mare that will or keep him by himself.

If he is successfully mounting, he could be dragging who-knows-what kind of bacteria into these mares. The last thing you want is for one of them to come up with some sort of infection and you be to blame.

A woman is closest to being naked when she is well dressed. -- Coco Chanel