Whether your fiance is around or not, I can almost guarantee your son, at some point in his life, will spank the monkey. Practically guaranteed. You can lock him away from the world and he will still do it. It's practically innate. The only real difference among men is the frequency/habitual nature, and we run the whole spectrum from habitual to almost non-existent. But we all try it at some point, even the saintly ones.
It's up to him, ultimately, whether he will stop or not. If he is a good person and makes you happy otherwise, my suggestion is try to work yourself into the ritual. For example, in most cases, he will probably find oral sex superior. I feel certain he'd be happy to return the favor.
The real question is are you getting what you need? Insist on getting your sexual needs fulfilled. If he has extra, um, time on his hands, leave him in peace. There are far worse places he could be putting it. Consider your relationship on the whole.