If he only took a lick, it should be okay. I would wet a small washcloth, and get him to chew on it several times, so that the ink will come off his tongue and mouth as much as you can. (I do this with Talon if she gets a hold of a pen) You'll be surprised how much you can get. I would also encourage him to drink, water,juice whatever you can. It will help to water the effects down.
He should be fine as long as he didn't get too much. Just watch him for any signs of lack of appetite, or behavior changes, if you see any take him to the vet asap!