I have a 25 year old half-Arab gelding, that I've owned for 22 years, he's been an easy keeper most of his life. I discovered Progressive Nutrition feeds about 3 years ago, and he hasn't looked better; I primarily feed the ProAdvantage grass formula (ration balancer). He has pretty good teeth, so is able to eat hay fine (the equine dentist will be here in a couple weeks to do everyone that needs it, including him; his teeth were so good we were able to skip the last two years, but he needs them done this year). [FYI - he's 15.2H and about 1050#.]
His winter and summer diets differ a little bit. Year round he gets 1# ration balancer per day and free choice grass hay/pasture. Last year I was able to get beet pulp pellets (they are a special order), so added 1# dry measure (then soaked) to his daily diet (this is all he will eat of the beet pulp, as he isn't thrilled with soaked feeds, I'll have to see if he'll eat more this year). This year he came out of winter a little thinner than I like so I added 3# alfalfa pellets to his diet and that helped bring his weight up. In the winter, I add 3# senior feed; with the ration balancer, the senior feed is primarily fed to add a good, easily digestible fiber source to his diet, as our hay is so-so (we grow our own, and we've had poor hay years lately; and grass hay is hard to come by around here, in the past he hasn't done too well with too much alfalfa hay in his diet, so I stick with grass hay). My BIL gave me two 32 gal garbage cans full of whole oats this spring, after seeding; so he's been getting about 2 cups 2x daily of the oats; he likes them and I think they are helping with his weight, so I'll continue with the oats. I also feed BOSS, about 1 cup daily seems to work for him.
If your boy is spitting out wads of hay (quidding), then he probably needs his teeth done. I dont' see my boy do this very often, but occasionally he does spit out a wad of hay (hence the reason for making sure his teeth get done this year). Hope this helps give you an idea of where to start.