Even once they're gelded, they'll try to mount.
The hormone levels will stay high for up to a year (usually a few months), and he'll think and act like a stud during that time.
If he's been used for breeding, he'll also associate mounting with extreme pleasure, and be conditioned to want to do it.
They'll eventually stop, at least most of them, when they discover things don't work like they used to, and they can't impregnate the mare after a certain amount of time (ask the vet; I can't remember off hand, but I know there can be residual sperm).
I once trailered two horses for a friend, a mare and a horse who had just been gelded (she didn't bother to tell me that) and had been used as a stud. I put the mare in the stock trailer, and started to lead him in - he bolted past me and hopped on. She stood, as she was tied, but it was quite an experience being in an open trailer with two horses mating (or trying, anyway). Not one I care to repeat!
The mares in the field will probably welcome his attentions when they're in heat, but you won't want to ride him with mares in season - too dangerous for the riders - until he's run out of lust and hormones.