I think he is just trying to shock his Nan - because he's not seriously consulting his Nan on this one, is he?
You could:
Be glad it is not one of those horrible earlobe plugs? Or his tongue?
Be glad it is not an enormous, visible tattoo being planned?
Suggest "maybe something more discrete darling", such as nipple, belly button and foreskin? (the out shock 'em approach)
Say you're thinking of having your nostril done and can you come along at the same time?
Remind yourself that most young people who have piercings get bored with them and take them out after a bit.
Send him some medical photos of piercings that have gone horribly wrong.
Suspect age guidelines apply to tattoos but maybe not piercings - it would have to cover ear lobes after all.
But that does not necessarily stop the DIY approach.