If the issue is confined to sex, then it could be that you two have a major difference in sex drive. In that case, the two of you need to have a serious talk, when you are both calm and not in the middle of an argument, to talk about how you are going to handle that difference without hurting each other.
But, often, when a couple has issues around sex, there are other problems in the relationship. When you are in disagreement, is he able to compromise? Give in at times? Does he understand that a relationship is about give as well as take? Do you get warmth and support and help when you need it? Or is life pretty much on his terms?
If the issues around sex are a reflection of your relationship, I hope you will take a big step back. In that case, I don’t think there is anything you can do or say to influence how your guy thinks about sex. He seems to think he is entitled to it whenever he is ready. He doesn’t seem to care whether you are really willing. For that reason, I think you are asking the wrong questions. The question to ask is why you are staying with a guy who is so selfish and self-centered.
Sex with a loved and loving partner is an expression of intimacy and togetherness. Both people need to be interested for it to be more than simply a physical release for one and an ordeal for the other. Yes, sometimes sex is a gift we give each other when we aren’t totally into it. But it’s unfair for him to regularly pressure you when you don’t feel well or when you aren’t feeling particularly close to him.
I wish you well.