SO Masturbation Issue

So I've never really had an issue with my SO masturbating, but recently it seems like every other time we hang out he has and it sort of ruins the mood for me. It just seems less special? If that's the right word....Anyways, I brought it up to him before and he has tried doing it less often and for the most part it worked. It use to be once a day but now I think it's once every three days. Then it seemed like he sort of gave up and started doing it a lot more often. I tried talking to him again because I'll be gone for a week and I asked him to hold off till I get back. He got defensive saying I shouldn't tell him when and when not to do it and the whole conversation didn't sort anything out. I just want him to stop doing it as often but every time I bring it back up he says the rate he does it is fine and that I shouldn't tell him what to do in the first place. Is my request to hold off for a week reasonable or am I asking too much from him? Also, do I have a say on him madturbating?