My friend's dad has some and he regrets where he got them and has always told us of the pain he suffered getting them in the first place and then having to live with them for the rest of his life until now when he went to get one removed and it turned out to be more painful than when he got them in the first place.
When my friend got married it was a really hot day and her father could not take his Jacket off and roll up his sleeves when the photo's were over because he was ashamed of his tattoos as they were in a very visible place.
His son went to get one and the guy had to stop half way through because he could no longer stick the pain and now he is left with a half a tattoo that means absolutely nothing so your son should go and discuss this with the artists and be aware of all of the dangers as well as the pros to having one, such as blood poisining, HIV, skin burning and so on.
But definitely do not say no as he will go anyway.