I'd be extremely worried! I take it that it is not hot enough for him to be sweating like that. Does your barn have fans? The thing that comes to my mind is colic (mostly because the only time my horse has ever perfusly sweated was because of it). Is he kicking at his sides or laying down to much...if so call the vet immediately!!!!!!!!!!!!!!!!!!!! OR!! Is he swallowing a lot, acting as though he is having a hard time breathing?? He may be choking. I'm not trying to scare you at all. It may be nothing. But if your horse fits the descriptions I was talking about call the vet immediately...if not, post in the horse health section and hopefully someone can help you.
It could also be nerves if your horse is alone in the barn. If it were me though, and my horse broke into a bad sweat without reason, I'd call the vet.