Tough spot, you probably just have to let him do his thing and see if he comes crawling back once he realizes the grass isn't greener on the other side of the fence.
Five years is a long time, guys get comfortable and used to someone. Even if he's dating someone, once that honeymoon phase wears off she'll be in constant competition with you whether or not she realizes it. Of course, theres a chance he was simply too unhappy in your relationship to appreciate the qualities where you win out. That said, you're at a point where your only options are to A. Ignore him and hope he comes around or B. Be proactive and get him back. I definately don't recommend calling him to tell him your feelings, thats relationship suicide at this point.
If you want him back you need to let him know he lost something worth having. Go out with other guys and make sure he knows about it. if there was an ex or someone he was always weary or jealous of make sure you get back in touch with them and make sure he finds out. Sometimes us guys need a little nudge in the right direction.