A spell would just be a band-aid to a bigger and more real problem in the relationship, in my opinion.
But about magic, perving at other girls when he's in a monogamous relationship is 1. internal, so it would literally take some magic to interfere with free will that deeply, and 2. physical, like it's not a literal wall of wind or electricity that's going to appear if he tries to sidle up to some sweet new thang 3. you could cast an anti-glamor spell around him, but if he's around guy friends egging him on then that can fall like a house of cards and 4. just a band-aid to a bigger and more real problem.
You've already talked to him...but still you don't trust him. Either you're too suspicious, or he's never earned your trust in that way, or both--but that, I think, is the real problem. Why does it take discipline to honor your perspective in the relationship? I'm more inclined to believe that it would take a breakdown of the objectification of women (including you) and empathy. Basically, just as an outsider to your relationship, what he does is not the focus so much as what he brings, which is doubt and distress. Can you really live with that, with him just being himself in ways that will never stop hurting you?
Otherwise, if you must: photo of your man, ribbon (whatever color), wrap the first with the second with your intention at heart. Say your spellcasting intention out loud if it helps. Aaand...done.