Some USA carriers could have their own rules about underage travelers. Check whether a notarized permission letter is required, for instance. For most purposes in the USA, anyone under 18 is a minor unless the kid has been officially emancipated from his/her parents, for example upon marriage (although alcohol is forbidden below age 21 and increasingly so is tobacco and related products).

It probably doesn't apply, but if your son wants to drive at all in the USA, an international license might be required.