I don't know about the U.K., but in the U.S., (may be some variations in state laws) minors cannot enter into a legal agreements - a parent or legally-appointed guardian must do so on their behalf.
(There's an exception for legally emancipated minors.)
So, technically, you have to enter into any agreements on behalf of your son.
Practically, I don't think there is generally any checking on this. Some registrars and hosts may ask about age, but I don't think it's commonly asked.
Generally, you have a credit card, (or perhaps PayPal account) you're good to go. Of course, a minor cannot have a credit card without a parent having signed for it. (Though I'm sure - given that they've been issued to cats, dogs, and goldfish, that more than a few slip through.)
An interesting side issue I've never really thought about - what, then, is the legal status of shrink-wrap licenses, EULAs, site terms and conditions, etc. with regard to minors?