Have you read THE SEX-STARVED MARRIAGE? (I believe Michele is currently writing "The Sex-Starved Wife"....so that will be great--I hope it comes out soon)
You're in a tough place because you otherwise have a good marriage....and you don't want to put too much pressure on him, and yet......it's hard to be on fire
The right antidepressant medication probably will help your situation as well as a full medical physical for him.
In the meantime, I think a good place to begin is to look at the EXCEPTIONS.....
When you DO make love....what goes on BEFORE you have an encournter...is it a setting, is it a certain mood, is it a better day at work?
I was in this situation a while back and it turned itself around on its own.....it seemed to have more to do about how he felt about himself....his life, his work, his body.
What are some little clues that you have?
sg Love is PATIENT, love is KIND, LOVE never fails / DB since 2001