I have no idea what it is that he's hiding, but some time around the time that things started to change in your sex life, five or so years ago, something happened, or he did something, or he thinks he did something, or... I dunno, but he's definitely restraining himself, and he's not telling you about something.
If you can get him to tell you honestly about it (or them -- it could be several things...), and get past all of whatever the consequences are for whatever it is, then your sex life will be restored or at least restorable.
A lot of "cleansing" suggestions have been made by other replies to this post, but I'll bet you another $5 that your hubby won't really get into doing them himself, so they won't ever really work. If he does have a genuine physical issue to go along with this, he won't willingly address the physical issue until the more spiritual issue of restraining himself from any sexual urges is addressed. And for that... he needs to open up and tell you whatever it is that he thinks he has to hide from you.