I like him. He won't put on muscle over his back til he puts on fat. Here's what I've done to get my very hard keeping older TB gaining:

Smart Gain 4--a great product from SmartPak for weight gain. Has probiotics to help with digestion--lots of time digestion is the issue.
Cocosoya Oil--no more expensive than corn oil but a little more nutritious
Senior Glo horse feed from ADM/Alliance Feed--very digestiable. I cut my horse back from 16 lbs grain a day (!) to 7.5 --3 ish pounds a meal.
An ulcer treatment, like G.U.T or UlcerGard might be a good thing to try.
Hay or pasture

My horse has gained 30-40 lbs in less than 2 months on this plan.