Cut the grain. Get a Vet out to check on him. Feed ONLY good quality grass hay, or grass/alfalfa mix. Keep putting it out free choice and let him eat as much as he wants. There are still hay shortages, so I won't argue. DO your HOMEWORK and buy your hay initially from feed store that also carries hay. Have your VET teach you about hay quality. (I like to get opinions from the horse people who are not selling the products.) Also, get AT LEAST a block of salt. I have free choice salt in a block in my horse's shelter's hay manger all year around, and we get decent snowfall here in central Illinois. If there is a University with an AG department, GET ON THE PHONE and ask questions. The University of Illinois Vet School Dr.'s will talk your ear off, but you can learn a LOT. There are also Community Colleges that have Vet Tech programs and you can get some help from them, too.
Your horse is young and Can recover. Even so, it ALWAYS takes more time to put back weight than it does to lose it.