I second the flax seed. Make sure you get the whole seeds and grind them yourself. It holds a lot more nutrition that way. It can also help keep a consistent weight.
I feed my horse 1/4 a cup of ground up flaxseeds once a day and it has worked wonders:)
Rather than trying supplements you can try linseed the oils in them help with a shiny healthy coat
I'd wait so you can see if his coat regains a shiny condition, otherwise you could just be covering up problems!
Also saves money that way