Appeal to something other than his basic instincts, but look good while you're doing it. Talk to him and engage him in conversations about something he likes, for example, music, books or travel.
Flirt with him subtly. Don't swoon, giggle or offer to film a porno with him on a first date like his usual girls. Beguile him, but maintain an aura of mystery.
Avoid asking, "How high?" when he says, "Jump." Womanizers tend to appeal to younger women or women with low self-esteem, and they are used to being catered to. A woman with her own life will intrigue him and begin to tame his wild side.
Treat his roving eye with nonchalance. This will be difficult at first if you really have feelings for him. However, all you can do is appeal to his better, more tame side. He will drop the horn dog act when he is ready. You are helping him now by acting differently than his normal bimbos.
Cultivate patience. It may take awhile for a womanizer to admit he's in a relationship, but if you and he really do have something in common other than temporary, animal attraction, it will happen.