His parents wouldn't sign him up under their credit card? I have two kids, one with a Nook Tablet and one with a Nook Color, and I set up an account under hte older one's name with my credit card. I set both of their Nooks so they need a password entered (which they do not know) to make purchases.
Another option is something my brother did for my niece who got a Nook HD for her birthday. There's a "child account." I'm not sure how it works but it sounded worse to me than the way I did it.
I also think a prepaid Visa gift card would probably work as a credit card for the registration? I'm not positive about that but it's a Visa number so maybe.