Alfalfa cubes are good supplement. They were excellent for putting weight back on my old arab mare the other winter when hay wasnt as good.
There are also several store-bought supplements and beet pulp, though you may have to mix that with something sweet for awhile for I have never personally met a horse that would eat it plain at first.
Alfalfa/Timothy hay cubes are the BEST for skinny horses, it's like they just reinflate the horse!
At our rescue all our old horses are on hay cubes, soaked (obviously) the 3 boys, that are your horse's size get 4 feedings of 2 quarts thoroughly soaked throughout the day, on top of their grain and good quality hay.
We have a TB stallion who just won't keep the weight on, this works wonders for him.
Honestly I'd be very concerned about his weight, but if you can't afford to have your hay tested and all that, which I completely understand, hay cubes just can't hurt if they're soaked (but not so long they ferment). They're also good for horses with bad teeth, so if it is a mouth problem this is a good temporary fix until you can get a dentist. If he doesn't gain weight on hay cubes there's a big problem, and a vet Needs to check him. A big belly can mean either worms or just that he's getting a lot of hay, but a hay belly isn't really healthy either. His food is just going right through him, not being properly digested. So if he doesn't gain weight on hay cubes get a vet out, he could have some sort of inability to digest grains properly too.
Hay cubes are so easy anyway, you just soak em up and feed em :)