The soybean meal is mixed with the grain. We got the grain mix recipe during a nutrition talk by Dr Tiznic, years ago. He was huge on feeding plain grains, not the mixes sold as complete feeds. Told us they always have “misc” on the list of ingredients, which could include anything! Might be floor sweepings at the end of the day.
We get our grain mixed at the local elevator. The current recipe is 250# of oats, 250# of cracked corn, to equal 500#. 50% of that is 50#s. So I end up with 550#s of grain total. You can modify the percentages on the corn and oats mix to suit your equines. When we had Western horses I mixed it 1/3 cracked corn, 2/3s oats because they didn’t need the fat from a higher corn ratio. Same 10% soybean meal added of corn/oats total. We do add vitamins and Selenium with Vit E, both being top-dressed because our soil has no Selenium. We top dress because we feed a small amount of grain once daily and top dressing insures everyone gets their correct amount. Our horses are pastured daily with room to move, run and fed good hay in winter. They get plenty of exercise along with most being worked pretty regularly.
You could top dress the soybean meal if horse is getting minimal quantity of grain or you prefer to feed a commercial grain mix. Start with a smaller amount like 1/4cup to see if he eats it for you. You can feed a bit more as things go on.
I do think more exercise helps hoof growth because it increases blood flow in the legs. If he is older, not rideable, perhaps just being ponied for more exercise would help increase blood flow. Can be done walking in a ring, though going down the road or trail seems to perk a horse up with new stuff to look at.