I'm not going to beat around the bush, every man I've met, been friends with, dated or had a relationship with has always talked about or been very up for sex in general. I know my circle of friends, however, does not represent the whole population in any way.
Therefore, I do know it's possible for some boys to just not be very into that kind of thing, that's all. Nothing wrong with it, and I know that you know you can't force or pressurise anyone into anything.
I'd suggest first talking to him and asking him if he just doesn't enjoy sex very much. Tell him you've noticed he doesn't want to do it, and you were just concerned that something was wrong etc. If he opens up and tells you about a problem, then you can discuss it together and work things out. (There's unlikely but small chances that something else serious is going on - he's homosexual/asexual/cheating on you. I wouldn't worry too much about these, hopefully if any are actually true he would eventually open up or drop a big clue).
If he gives you no answer or an answer you can't work with, and are still unhappy, then as others have said perhaps you just aren't compatible. If differences are met with little problem in a relationship, the relationship can move on fine. However, if differences cause upsets/hiccups in a relationship like they seem to be doing here, then it might be best to end it if it's not working too well. You can remain friends if you wish. That depends on how close you are as a couple and maybe how long you've been together.
I hope you work it out.