He likes grooming and it can be a very special thing for you and the cat to bond through... but I've only thought of you helping him groom using a brush rather than your tongue. Hmm. I'm really really really really hoping and also assuming that you're just licking his face/head. I'm sure it's fine. If he doesn't seem upset, he probably isn't.

And make sure you're not chewing gum. lol.