Whilst his mum and sister might encourage it, it really ishis choice and if he didn't want one, I'm sure they wouldn't strap him down to the tattooist's chair, would they?
People do have some silly tattoo's yes. But people also have some nice ones. I think perhaps you could highlight to him the social consequences of getting a swasticka, or a tattoo on his forehead and hope that he takes your advice on board.
At the end of the day, it is his decision. And whilst you could throw him out of your house, I think that would do huge damage to your relationship when what you are doing is imposing your views on him, which is wrong. Especially when he's 21. If he was 15, I'd probably agree with you wanting to stop this, but he's an adult. He has a right to make his own choices.