It seem to me like the problem is much larger then just your sex life, but that this became the battle field of the relationship. It's obvious that this didn't start over night, nor will it be resolved over night.
I he can masturbate regularly, then that basically rules out a physical problem. I think that maybe things just changed between the two of you, that there's some anger that makes him want to stay apart from you on anything intimate or sexual.
I think that first of all it's important that the two of you stop acting as if everything is like it used to be and acknowledge the fact that there is a problem. The problem might not be just with him, but as a couple, if one wants sex and the other don't then that is an issue that should be addressed. It seem that the two of you just avoid it and pretend as if business are as usual.
There are al ot of things you can do to resolve it, whatever the source of the problem is, but first of all, there should be willingness on both part to address the problem.