Rachael, you're going to have to rape him.
Actually, I'm all out of good suggestions. Maybe sex is more of a chore for him. Masturbation is easy, but sex takes effort. It's hard enough to get excited about it when you have your partner breathing down your neck about it. I'm not blaming you, but it doesn't always help to talk.
This guy needs some peer pressure. Get him some friends who like to brag about how great their girl is in bed. He'll appreciate you more then.
We never like to encourage people to cheat, but if I were in his position, I would be worried about my girl looking elsewhere. I mean, somewhere along the lines of a relationship agreement has to come sexual gratification. A lot of people like to shove that to the back and they consider it just one of those extras about a relationship. Well, they're wrong.
Anyways... yeah.. rape him.