Seriously get that boy some pasture time. I own a TB and without grass, he becomes a hard keeper. Remember hay is also your weight builder as is hay stretcher. You can try adding a QT of hay stretcher am & pm to his grain. Nutrena has a High fat performance grain that mine does well on, I dont go any higher than 10% as it quickly turns him into a nut case.
Before you do anything have him checked for hindgut cecal acidosis.