My husband tattoos. He said that sometimes, inner lip tattoos last for a very long time; other times, they go away.
I can't believe that they'll let him get tattooed at 15! I know that in NY and CA, the minimum age is 18.
But hey. If he is mature and feels ready to get it.. I am not against it if you go to a legit shop and everything is done cleanly!

I can't give an idea though, without knowing him. Should be something that is just "him."