Like you, I am rehab-ing an OTTB who left the track with “track feet”: thin walls and soles, a back injury and very underweight. I found that he needed certain nutritional building blocks as well as additional energy in order to build healthy tissues. I suggest you start with a good hoof supplement that provides trace minerals (zinc and copper) and biotin. My guy’s ribs began to disappear about three weeks after I started him on a hoof supplement. He tested low for blood levels of selenium and vitamin E, so I supplemented those as well. But do have him tested for selenium before supplementing. Flax seed (source of omega fatty acids) also helped his skin, hair and hooves and seemed to help with weight gain. I also found he needed extra protein in order to lay down muscle. I gave him one pound of alfalfa pellets (about 80 grams of protein) and an essential amino acid supplement (lysine, methionine, threonine) to boost the quality of the crude protein. In a matter of weeks, he laid down enough muscle in his back to stabilize the old injury and became much stronger, allowing for additional exercise. Also improved the soles of his feet (probably due to increased protein and calcium in the alfalfa) and the inflammation in his feet finally went away.
Every rehab is different, so you have to resort to a bit of trial and error. If any of one the above doesn’t work, stop using it, he may not be lacking that particular nutrient and you don’t want to waste your money on something he does not need. But also remember, if any one nutrient is lacking, he can’t build healthy tissue, so you might need to look for other deficiencies.