Maybe I‘m wrong but it sounds to me that he has played away and has caught something nasty,and that‘s the most likely reason he won‘t have any sexual contact with you!
I hope I am wrong,for your sake,but it just seems strange that your sex life was quite healthy between you,then to all of a sudden stop? That would ring alarm bells for me too.
He‘s probably finding it very hard to come clean about whats on his mind right now,and if he hasn‘t played away,he shouldn‘t find it that difficult to be open and honest.
Things don‘t just go off the boil over night if we havn‘t been going through a bad patch,we usually see it coming over a period of time if we have problems in our relationships that doesn‘t involve anyone else!
Goodluck.