What feed brands do you have available?
I would pick the highest quality name brand and buy their "grow start" or whatever that company's feed name is for weanlings
I wouldn't feed anything more than a quality feed for awhile but at some point, I think adding Omega-3 Horseshine or someone's flax would help his hoof and coat quality a lot
I know someone who took a TWH colt that literally got weaned at the auction because the beatch that bought the mare said she didn't care if the colt died in the ditch somewhere
He never fed that colt anything, just turned it out to pasture with his older geldings who thankfully babysat the little guy.
I hadn't been past his house for an entire winter and one spring, about 4 years later, I saw this big, pretty sorrel standing in the pasture.
I called him and asked when he got the new horse. He laughed and said "that's Cotton -- turned out pretty good sized after all, didn't he?"
Cotton grew to around 14.3H or 15H. The TWH in my avatar has never been starved or abused and he is 14.3H.
My point to that: You have him now and with great care, you will probably help him recoup his growing losses; it will just take him longer to grow
Kudos to you for giving him a home