I had a tattoo done when I was 16. Wasn't asked for ID at all. I did try and make myself look a bit older.
The fact remains that I wanted a tattoo, I went in and had it done on my own steam. If he had asked for ID, I had some fake ID ready. If he'd sent me on my way, I would have gone to another tattooist, and another, and another... until someone agreed to do it.
Perhaps go and see the tattooist, tell him your stepson was underage. If he thought he was underage and didn't bother asking, it may frighten him into thinking twice about doing it again. It may have been a genuine mistake - you really don't know for sure.