If he's happy with the situation, you should be happy. But, to make him happier . . .
From an older guy's viewpoint - you might do well to keep experimenting periodically. In my sexual career, I have run into a lot of guys who said they once claimed they were total tops, but later on became quite versatile when it came to fucking - after they had more experience. For some, it was denial - they could not admit to themselves that could like another guy to have such power over them, and did not want to feel submissive. From my jaded viewpoint, it's hard to believe that a 21 year old already knows all the things he can be sexually - not impossible, but not likely. On the coasts, guys your age are all doing drugs - like eXstacy (or worse things things like meth, that I could never recommend) that allow one to broaden one's horizons. Or acid, or merely poppers. Maybe you're one of those guys that is incapable of anal pleasure - there are quite a number on Rj say that's the case for them. But you have an awfully long way to go before the end of the road - so you might occasionally experiment to see if you might change. Anyway, you asked.