Im 18 and on my brithday I just about got a tattoo even though my parents didn't want me to. My mom wasn't pushy, she didn't tell me no. She gave me the choice, told me she would be disappointed but would love me still the same. Obviously it's past the point of getting him to listen to you. But to prevent further rebelion from your son, as a pretty stubborn and rebellious 18 year old myself I would say just love him. Let him know you are disappointed in his decision but that doesn't change how much you love him. Dont get angry and don't punish him. If it was that important to him, let it be. Sometimes, what it really comes down to is him getting a tattoo now bad enough you want to lose your son over? Cause eventually, if you get pissed about this, you'll slowly push him farther and farther away, and deeper and deeper into rebellion. Also, as a parent who has more wisdom and experience than your son, just know that one day he will probably regret it. He will feel stupid for not listening to you, but he will ALWAYS remember that you loved him anyway.