I might be completely off with this, but did you have bot flies?
I went through something similar years ago with my colt, same age. He wouldn't eat his grain, only nibbled at hay for about a week. Nothing else wrong, no signs of colic etc.
Vet said it could be migrating bot larvae. They get ingested when they lick at the itchy eggs, larvae hatch and bury themselves in the mucous membranes in the mouth, emerge after a while and being swallowed to attach themselves in the stomach lining. The emerging can cause enough irritation for some, mainly young horses, to go off feed.
Since that happened, I worm with Ivermectin right after the first frost, if there were bot flies and haven't had that problem since(15 years)
Last edited by deserthorsewoman; 01-14-2013 at 12:21 PM.