He will be fine.
Just make sure he eats lots of hay and drinks lots of water today, it may upset his tummy if he got alot, but if he was willing to lick the spot, it couldn't have been that much.
In the future, I would just leave it on if its in the "wrong spot" then wash it off in a few hours, at least allow the revolution to do its thing so you dont have to re-dose for the month.
I like to apply it at the very base of the skull kind of behind the ears/above the shoulders, in multiple spots.