I'm with the girlfriend, try something new and exciting. Turn things up a little. Like porn or a toy. Also try girl on top and you do the work. He wont be using his back that way. I dont know though my guy loves to get me going before he gets his, its a turnon for him to get me turned on first. Most of the time he is very conciuos of my needs- cant spell-But there are times where Hes not into it and we just kinda get it over with.
It sounds like he has something else going on, he could feel useless because he cant satisfy you, Im sure he can tell the difference in your relationship also. Its probably hurting his pride and feelings of being a man. Daniel gets like that when his back hurts him.
Ask him about therapy. See if he would be willing to try it. If he really wants to save the relationship he will do it.
daniel has to take sleeping pills for bed. He takes them at like 7 and after he takes them hes too tired to even stand let alone kiss me. It sucks cause I get off work at four, then we get home make dinner clean and take shower and by that time hes passed out. Theres no time for real sex except on the weekend. So I know how it is to not have it all of the time. It sucks.