Maybe you could try giving him extra attention in other areas, such as sharing an interest of his or talking about or participating in something he likes to. If he feels like he's getting your attention outside of bed, perhaps he won't ask for sex so often. But maybe you're already doing that and he still wants it all the time. Maybe it's a power struggle thing with him as others have suggested. Give him the illusion of power in other areas and maybe he'll back off.

Can you help him "satisfy" his urges in a creative way that doesn't involve as much physical involvement on your part? Or, tell him you don't feel like it at the moment and give a description of what he can look forward to when you do feel like it. Maybe you can convince him of the pleasures of quality over quantity. And be sure he knows what puts you in the mood so he can make an effort rather than springing it on you, so to speak, when he suddenly feels like it.

I think differing sex drives is right up there with money when it comes to tough relationship issues. Good luck...