If he won't go to therapy with you and he seems more withdrawn, take a look at how his life outside of the marriage is: work, time with the guys, etc. He could simply be stressed, which will lead to a decrease in drive to have sex. There is also the possibility that he's committing adultery behind your back (And for your sake, I hope not). The best thing to do would be to sit down somewhere where you won't be distracted and have a talk about this. Ask why he seems so disinterested and whatnot. 

I hope it works out in the end for you :) good luck!