Start, of course, with the chiropractor to rule out any pain problems due to misalignment. I would also check ALL tack, saddle, bridle, everything to make sure theres no pain coming from that.
Is he shod at all? Rock in a hoof, or sensitive hooves perhaps? When was the last time his sheath was cleaned and teeth floated? I had issues with my gelding because of a unclean sheath and teeth in need of a float. Now he just has some serious barn sour problems and will bounce, hop, and bolt JUST to get back to the barn.
What kind of bit are you using? Also, how frequently are you riding him and were do you ride the most?
How is he on a lunge? How are his ground manners? How does he respond to ground commands like walk, stop, turn, back up?
Wait! I'll fix it....