Lots of transitions help engage the hind end. Trot poles also work well. Once he's going over trot poles consistently well, raised trot poles. My OTTB didn't used to use his hind end either. After tons of the above mentioned work, combined with trail rides in the woods to get him thinking about where to put his feet, he is now trotting small cross rails instead of jumping them since he had now figured out that he has 4 legs and not 2.
From a rider standpoint, make sure you are not riding in such a way as to hinder his movement. Make sure you're centered over his center, not leaning over and loading up his shoulders. To test whether you're centered, trot on an insanely loose rein so you have nothing to hang onto, and post so that you are down for one beat but up for two. If you're not centered, you'll either tip forward or fall back. Initially do the poles in a light seat so as to stay off your horse's back while he's figuring out this hind end thing. Also make sure to use heavy poles. You don't want them rolling when he inevitably trips on them or try to plow through them.