1) How old are you? If you are underage this is illegal
2) Even if you are giving him what he wants, what he is doing is wrong and he is coercing you into sexual acts. This is a form of sexual abuse.
3) I think you need to find a way to be safe. If you told your sister how would she react? Do you have a safe person you can confide in. Going to the authorities is a personal decision but you might want to highly consider talking to a safe person so you can find a resolution that will lead to your safety.