For weight gain: corn oil! In all the cases i've heard, it's works wonders. Ask your vet how much he would take or look it up online. Corn oil also helps a great deal with the coat.
Now, if a horse is healthy, there coat should be too. That's one of the main signs that a horse is healthy: good coat.
So his coat might improve a little now that he's wormed.
If you're looking for something just for the coat and not as much for weight gain, I suggest feeding him 1/3 cup of flax seed -- the oils in flax seeds are really good for horses and it makes their coat beautiful. Feed it to him with some grain (if he gets grain).