I vote help him. He's 18 so bottom line he can get one with or without your permission. At least if he know's your behind it you can offer suggestions and perhaps help find a good place.

Just a suggestion, do your homework (ask friends, etc.)and pay a little more if necessary. My brother had a tatoo done at one those ?? places for less and has never been really happy with the work. I had a tiger done but did my homework first and went to a shop and artist that was extremely well known and even published in magazines. Everyone I knew recommended this guy. The tiger looks real it's so good. The other thing is since he's young and has no idea what type of job he'll end up with suggest putting it in a place that's easy to hide, believe me it will make things easier down the line.