Go to your local police station, tell them the circumstances and make sure they're aware of the infrequently used "Tattooing against minors act 1969" (yes, it exists.) It's an old statute but if I remember my training rightly (so long ago!) the only real defence is that the person didn't know the age of the person being tattooed. In this instance, being related means that that defence is most likely unavailable. The punishments are paltry but they are punishments.
Meanwhile make sure your son is caring for his tattoo. That means applying moisturiser to it, not picking at it etc. It's his body for life.
Meanwhile make sure your son is caring for his tattoo. That means applying moisturiser to it, not picking at it etc. It's his body for life.