There could be a bunch of things going on there.
He very well could be a gelding...my gelding mounted mares all the time. He could have just been gelded late and still like the girls
Another possiblity is his testicle did not descend out of his abdomen, making him APPEAR like a gelding. You might want to have that looked at. He could still breed if that is the case, and also could lead to some health issues.