You could probably still fairly safely scrub him even though it's getting chilly.
If you don't have warm water at your barn, bring some hot water with you, and mix with cool water from your barn. then scrub him and then sponge the stuff off. Blanket him for a while, while he dries, and then apply the spray when he is dry.
You could also try Vetricyn...it's really expensive, but I've heard ALOT of good reviews on it from friends, and family who have used it. That you just spray on, no need to scrub prior, although I'm sure you could, and it wouldn't hurt.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."