Before you put your horse on selenium get a VET's opinion first. Horses do need selenium and it's verrrryyy important but if you give too much you'll have even more problems. I can't remember the selenium toxicity signs, but any good vet book lists them.
Before you give any vitamin/mineral supplement, you should take the horse to an equine vet and get their opinion.
I hope he gets better! Good luck!
What doesn't get rough, doesn't get challenged. What doesn't get challenged, doesn't improve.