Drafts don't always do well on grain diets. You might look in to feeding him a combination of beet pulp, alfalfa pellets, and a fat supplement (rice bran, flax, corn oil, etc.), no grains. Once his weight is back up, you can reduce or remove the beet pulp and reduce the fat.
Start with 2 lbs of the grain you're giving him, 1 lb of beet pulp (dry weight) and 1 lb of alfalfa pellets. Make sure the beet pulp contains NO molasses. Give that a few days, then add 1/4 cup of corn oil or rice bran. Give that a few days. Then reduce your grain to 1 lb, increase bp and ap to 2 lbs each, and increase the fat to 1/2 cup. After a week, remove your grain, increase the bp and ap to 3 lbs each, and fat to 1 cup. Leave him on that for 2-3 weeks and see how he does.
He "should" be able to maintain weight on 0-1 lb beet pulp, 2-3 lbs alfalfa pellets, and 1/2 cup of fat.