Weight builders don't do much for TBs or other "hot" breeds that are "hard keepers".
Did you ask your feed store about Alfalfa pellets? Did you ask them about rice bran or flax? These are things that just about any feed store has, or can order for you.
Did you ask your barn manager about giving him more hay? I would increase his hay by 50%, and eliminate the grain/feed and corn. Have her feed him alfalfa pellets, rice bran or flax, and a vitamin supplement. Add some water to make it all stick together, or some corn oil. Within 30-60 days, you'll have a whole new horse, trust me ;).
If she will not increase his hay, then I would add shredded beat pulp to the alfalfa pellets, for added calories, protein, and fiber. Most all feed stores have beet pulp. Just make sure it's the kind with NO molasses.