Not sure if I can help but I’m willing to take a stab at it. I would start with what you know he likes, so he likes to look out the apartment window at changing girls. Is there one woman in particular that stands out, if so I would use here as the bases of the 3rd woman in the story. Or you could have him first see this woman in the story through her widow one night as she is changing.

Mind control has never been my thing but maybe you two go to a conference on hypnotism that teaches you how to unleash a persons hidden desires. From you description of your and his interests it does not sound like you want this story to be to rough so it should not be two hard to draw out her (the third women’s) desires. Maybe she has a bad habit that needs breaking and hypnotism and a little kinky 3-way sex would be just the thing to do it. Maybe she agrees to be spanked when she fails and is rewarded in other ways when shew has been good.

Anyways those are just a few ideas, and good luck!