I agree strongly with Jen. Before you do anything else, worm him with something that will kill encysted strongyles and tapeworms. I can't stress this enough. Even though he's on a regular deworming schedule and even if fecal egg counts come back clean, he could have a high parasite load. I lost five horses once to a rare parasite, a type of small strongyle, that had encysted (buried up in the intestinal lining to "hibernate"). When they're encysted, they don't lay eggs so all fecals done can come back clean ~ absolutely zero. This was the case with my horses. One horse in particular died twelve hours after a sample was taken that tested clean. BTW, ALL strongyles can and do encyst ~ rare ones, common ones, small ones and large ones.
So I strongly urge you to rule this out by getting him wormed with the fenbendazole multi-day regimen to ensure any and all encysted parasites are gone. It won't hurt him, but not doing it could hurt him. Then worm with something that kills tapeworms (or does the fenbendazole get the tapes too, Jen? I can't remember).
If he doesn't start gaining after that, I'd have his teeth looked at again. Every year isn't that often ~ it's actually a good practice to get them done that often, atleast looked at to make sure they don't need it. Some horses even need it more often than that, even younger ones ~ it's not common in younger ones, but it's possible and does happen.
And try to find a veterinarian who specializes in equine dentistry. Most vest don't spend much class time on dentistry, so while they know how to do a basic "float", they very well may not know how to fix severe hooks and other abnormalities.
This is something else that one may think, "Well, his teeth are fine. If not, I'd know something." Not so. Earlier this year, I spent $1800 getting ten of my horses done by Dr. Clay Stubbs, an EXCELLENT
veterinarian who specializes in horse dentistry (he even has multiple patents on dentistry tools he's designed himself) and it was worth every single penny! I was amazed at how some of my horses' teeth looked. I never would have had any idea. The ones who dropped their feed were obvious, but there were a couple who needed it worse than the feed-dropping ones despite them not dropping any feed or weight at all.
Just the education I got from Dr. Stubbs was worth more than what I paid him. He let me look up inside my horses' mouths, pointed out things to me, told me how they happen, showed me what they do to the horse's cheeks and tongue and told me how the horse would show it with his actions. On one little gelding in particular (one of the ones whose teeth needed work badly but didn't drop his feed) he said, "This guy probably throws his head around a lot, even when you use a hackamore, and doesn't like going to the left." I'll be darned if he wasn't right!!! And all that stopped after he healed up from Dr. Stubbs' working on him.
So, if the worming doesn't work, check his teeth. If it's not his teeth, then
start looking at feed supplements (btw, I LOVE corn oil and SWEAR by it ~ wish I had my before and after pictures to show you). Doing these two things first could save you a lot of money since almost no amount of feed can put weight on a horse with a high worm load, and the amount and type of feed needed to put weight on a horse with crappy teeth will break the bank! LOL!
And let us know how it turns out!