Can you get a suppliment to the BO to give to him? You could ration it out in daily amounts in a ziplock baggies, then give them to BO. That's what I do. Sometimes like DuMor (TSC brand) Hoof, or any flax seed / biotin /vitamin A type suppliment wuold do him a lot of good, I think.
I have a little TB filly that I'm working with who is in the same type of situation as your guy, but I have her on 24/7 grass. Bad, abcessed, underrun, thin soled feet, underweight, and poor internal health. I have her on a Nutrena feed, DuMor UltraShine, and 5 lbs of alfalfa pellets daily, and she's starting to look a lot better. I'm keeping her barefoot though, and having her trimmed every 3 weeks rather than the usual 5-6. She didn't have as many cracks but they're already looking much better. She was VERY lame for a while because of her feet, but constant moderate exercise, good hoof, and good trims go a long way!
Purina is ok...I had Kenzie (TB filly) on Purine Equine Junior for a few months but switched over to Nutrena because the purina feed has quite a bit of mollasses in it, and I kept finding pieces of corn in the feed when there shouldn't have been any at all. Do you have Nutrena product access? If you have a tractor supply, you should. I'd probably put him on Nutrena Life Design Senior. It has flax, rice bran and beet pulp in it, so he'd be getting a little 'boost' from it.