It's his choice, please don't listen to the small minded people who are saying that they're tramp stamps.
Yes, this is the colloqiual name for them, but the tattoo doesn't make the person a tramp, the person makes the tattoo LOOK trampy.
And this is the twenty first century, gender equality and all that jazz. I know a guy with a lower back tattoo that means a lot to him, and it looks amazing. If this is what your son wants, if it has meaning to him, let him get it. :)
If your son has wanted this for a while, he probably won't regret his decision, and if he does, that's his fault, and he can foot the cover-up/laser surgery cost.
If he's soon to be 18, you can't really go further than give him your input and opinion.