I would be careful trying to overfeed a young horse for the risk of development issues. We feed our youngsters quality hay and a ration balancer. That on top of unlimited turnout does the trick.
Powerstance by Stance Equine. It's very safe for the gastric tract and it's all coconut so even the hooves will be absolutely fine.
I used to put some vegetal oil in his food, but that wasn't interesting, he gained some weight, but not that much. I put then corn every morning and every evening also in his food, and the result is amazing! And that's also interesting about the money :p
Make sure he's ok (vet, dentist, etc) because that can be alarming to have a foal who doesn't get some weight! But don't panic, the foals are growing up really fast, and sometimes they don't gain weight as fast as they grow up! If we can see too much the ribbings, be careful, if we see it but as normal as usual, calm down, he's growing up :)