Hey wait! What about just a regular ftp account? He could get around the "shared files" problem by just logging in himself using the id/password that he gives his parents...
(Apologies for referring to you in the third person, AMD)
Oh, and AMD, any pure web-based solution would run into the PHP file upload limit. Getting around that is doable, but probably requires more work than it's worth for your application (but you tell us).
Free unique IP and $67 off with promo code [color=#CC0000]FLENSFREEIP67[/color] or use [color=#CC0000]FLENS97[/color] for $97 off. Click here for more options