Along the lines of what Fancy said, maybe add some oats to his feed...or use a complete feed with oats (like Complete Advantage). My horse gets far too much energy with even the slightest hint of oats, and she's already sensitive and explosive when in the wrong hands. (It's a very night and day reaction.) But, also, like she said, I've seen many TBs that were laid back too.

Fancy, with the stringhalt issue, they weren't my OTTBs, so I don't know how they were being fed or supplemented. Mine never had stringhalt, but several people I know had OTTBs that had it...funny thing was that they were also really talented show jumpers with amazing ability to jump like gazelles. I can think of five offhand. Could just be coincidence.