Leaves you between a rock and a hard place. You have to choose between your son and hubby. I imagine it will cause a lot of friction in the home if he does get it. Yet, at 18 he is old enough to make a decision.Would he be willing to wait another year? If not, everyone will have to meet halfway.
I never was a tatoo fan, so I would also have said no to my kids had they asked. I guess if it's in good taste and somewhere hidden, it would be ok. My main concern would be what he could pick up. No matter how reliable the place, there's a lot of risk, take a look at what you pick up from needles at the hospitals.