I've written a few articles on this topic geared towards men. I don't know if he'd read them, but here are some links:

https://medium.com/long-after-the-thrill/youll-have-to-do-it-when-she-s-gone-0306ce966d53

https://medium.com/long-after-the-thrill/why-she-wont-have-sex-with-you-anymore-1a7b8a8b295e