To stay healthy, you should only be riding him very lightly this winter. Big geldings have knees close around 3yrs old with their hocks closing at 4.5-5 yrs old. Heavy training should wait until after 5yrs old.
As for feeding, if he weighs 1,000 lbs, he should be getting 25-35 lbs of hay a day. If your bales are standard grass bales weighing in at 55-75 lbs, that's half a bale or more. Since he's young, I'd give him as much hay as he'll eat.
For added protein, I would give him some Alfalfa, either hay or pellets. One small flake of hay should be enough, or 2 two qt feed scoops a day (about 3-5 lbs).
To round out his vitamin/mineral needs, I'd go with the supplement Balance II
, Uckele's Equi-Base Grass
, or Triple Crown's 30% Supplement
). The first three are just vitamin supplements. You'll want to mix them with some of the alfalfa pellets and a dash of oil if he sifts it out, or some plain whole oats. The TC 30% is a highly concentrated pelleted feed (a ration balancer). You feed 1-2 lbs a day for full nutrition.
I prefer to feed vitamin supplements or a concentrated ration balancer along with forage based products because it's healthier for the horse. Ingredients like corn, wheat, grain sweepings, and molasses have negative effects on horses; they can can metabolic issues, behavioral issues, colic, and founder/laminitis. It's just safer and more natural to stay away from sweet feeds or feed pellets. If you don't feed at the levels recommended on the side of the bag, then the horse isn't getting all the nutrients it needs. If you do follow the recommendations on the bag, the horse is getting too much starch and sugars. It's a no-win situation.
Since switching to a forage based diet my hard keeper keeps weight on with a LOT less food than he did before, my feed bill is LOWER, both horses have great hair and foot growth, my mare's heats are less noticable, both are less spooky/fidgeting in new situations, and they both have a better work ethic in the arena. And mine is not an isolated situation. Our WHOLE BARN switched to this diet at the last boarding facility I was at. All of the horses showed positive improvement.
If he needs more calories, add plain whole oats to his diet and come corn oil or whole flax seeds (working up to 1-2 cups a day for either). The oil and flax are good fat sources for concentrated calories.