I find it pretty stupid whenever there's a strangely ridiculous topic on the BLF, most of the users freak out and start thinking that the OP isn't being serious afterwards.
A lot of the users also treat it as it's not even the OP's choice in said subject.
Seriously, the only thing you can say is that it's not something that is suggested or recommended and that you discourage it or something, not "DO THAT NOT THIS AND SHUT UP". Though, you could just ignore the topic, or even possibly contribute to the topic!
It's his choice, if he wants to do it, let him do it. If he doesn't like it later in life, that's his problem. Don't turn his problem into yours.
Even though a tattoo would be less concerning in the future, it's still his choice.