I have 2 tattoos. They are in peek a boo spots. They only show if I want them too. I don't know what your son's pain threshold is but the outline hurt like heck. I think your right that he should start out small and see if he likes it. The color may not show as he envisions it and he may decide tattoos are not for him.

A whole sleeve is a bit much and I'm not sure they do it all at once. It's a big decision to make and maybe you could bring up the topic by saying someone you know or know of is thinking of getting a rather large tattoo and ask him some questions or just voice your concerns or opinions. He could be doing something worse but you never know what biases or problems he could run into in the future with a tattooed arm.