If you go to the ABC forum and search it, you can find topics where they discuss the use of artificial lighting in the winter to keep them fertile. It has something to do with the number of daylight hours they are exposed to. I know in horses that mares will go through a period of "anestrus" (I hope I spelled that right) if they are not put under lights as breeding season approaches. Look at the post by Beth C "Breeding pen size and construction". John Blehm posted pics of his breeding setup that shows what I would think would be very inexpensive use of rope lighting. Of course, those won't work in a flock set up. But if you have electricity in your coop, you could install a timer on your lights so they come on an extra (4?) hours a day until the daylight hours are longer naturally.
Point is, if he was fertile a few months ago, this is probably your culprit. It's an easy fix, but not necessarily an immediate fix.
Cindy