I can’t make my boyfriend cum

Me and my boyfriend have together for about half a year we are each other’s first and everything so far has been great. He loves going down on me and is very attentive, so much so I cum multiple time every time we have sex. The problem is that I am unable to make him cum. He insists that’s it’s not my abilities or lack of his attraction. I believe him as he has cum to me multiple time while masterbating. But I feel bad that he always has to finish himself. He has concluded that the problem stems from his masterbation habits and he has decided to stop masterbaiting for a month or so. Has this problem happened to anyone else before and if so how did you and your partner fix it? I feel like there’s something more I could be doing to help him as I really care for him and want to pleasure him as much as possible! Thank you.