You could ask him to stop, but that doesn't necessarily mean that he will. Masturbation is fine and normal to engage in, but if it stops someone from being intimate with their partner, then it usually becomes a serious problem for the couple...
Since you've talked with him about your concerns, has he tried to become more accommodating regarding satisfying you, or not? If he hasn't, that is a definite red flag that he has no intention of changing, or just isn't interested.
In my opinion, if he doesn't start to show any interest in your sex life, it would probably be a waste of your time to continue with this relationship. I understand that you love him deeply, but you don't sound very happy with your boyfriend. Healthy relationships thrive on mutual satisfaction and emotional support; if you're not getting that from this relationship, maybe it's time to re-evaluate why you stay with him?
Did you try to find out if he is watching porno?. Only masturbation does not give you enjoyment as intercourse. Probably he is doing it with porno watching. This is more serious than only masturbation. If he cooperates, you may visit sex expert. If he accepts the visit to professional , then you are sure that he means well. Otherwise he is cheating you..