How deep/large is it? Depending opon the size, I'd ad TucoPrimm to his feed (antibiotic), or penicillin (ask your vet, how much depending upon weight of horse), possibly both if it's serious. Vet advice is great, even if it's just a phone call.
Dock Leaf and Plantain is also very beneficial (100% natural, pick it from your yard and boil it about 5 minutes to sterilize) chopped up in small pieces and put on the wound as a type of poutice. Use a clean, thin cloth and a bit of vet wrap and leave it on 4-5 hrs before removing it to let air dry. I do this daily, it is amazing. Aids in a speedy heal-up, helps reduce swelling and proud-flesh. I could almost swear by these two plants! (Though my vet is oblivious to them both lol
Do you have any photos you could post of the wound? We might be able to help you better. I deal with alot of cuts and scrapes, both very serious and then the not-so-serious ones...
Good luck, I wish him a speedy recovery, and find out what that vet says
