I second the ration balancer. Be careful not to overload his system with a whole bunch of new stuff. Small amounts fed several times a day is the best way, working the amount up with time.
I have rehabbed several very sad looking horses before. What has worked for me is good pasture at all times, with hay available at all times and water, of course.
Beet pulp, Cool calories, Flax seed oil, a good quality pelleted feed (so you can soak it all) and a ration balancer.
If you can get your hands on the brand Triple Crown, the have very good feeds as well as a ration balancer.
Also, post pics! We are all really nosey here. LOL
Proud owner of ~Mana: 6yo Arabian gelding~Pearl 13yo Arabian~Danzer 14yo Arabian mare~ Tiny mini filly