We don't know what your feeding grain wise but all horses should be fed by weight not volume. Try worming him to start this new process & getting his teeth checked
Being your horse's weight should be at 1,100lbs. You should feed him like a 1,100lb horse! He possibly needs 20-25lbs of feed a day, less grain more hay & vitamin supplement feed, with a alfalfa pellets or a beet pulp mash. vitamin supplement Feeds like... Purina Enriched 32 or Blue Seals Sunshine plus for example, this way you don't have to add anything to his meals, its in the bag! & you feed less of it.
If not try using what you already have...
What exactly are you feeding your horse twice a day?
12%, 16% is it a sweet or a pellet?
Maybe try adding to his already existing feed schedule::
A handful of sunflower seeds - helps weight gain & shiny coat in his meals.
Great the horse is out on pasture bales for 24/7 also he is getting Vitamin D from the sun! But is that round bale good enough hay and is he eating enough?
Try to bring him in for a extra feeding of a 1/2 scoop of alfalfa pellets at lunch time & a fresh pale of water (or hours before dinner) & drizzle 1oz. Coco soy oil on his pellets.
If he doesn't like the oil taste add a packet of oatmeal instant start stuff or do it anyway and we all own corn starch! Add a pinch of that for 15% calcium & a little water to let the pellets absorb. Then let stand before he eats it.
Be creative, smash carrots & cucumbers, mix a shot glass of fresh ground flax seed in the mix. Give your horse vitamins to allow his body to function properly. Add some salt so he drinks more fresh water.
Good quality foliage is key for your horses nutritain. Giving your horse the right vitamins, hay/foliage & water is key for his health.