I think Askpapabear might be able to help , www.askpapabear.com

What I would do is act like I've obeyed them and then entirely fucking ignore them. If my parents told me to stop doing something that I refuse to, that's exactly what I do.

It is hard to give advice for one so young, but I would agree Saska's idea is one way of going about it. Pretend to comply and just keep this one facet of himself a secret, at least until he is older and out on his own.
Or perhaps try and defuse the situation by trying to explain it in more innocent terms, like a form of cosplay. Showing them it is just another form of things that conventioneers do.