Honestly I started freelance designing in my teens. I learned how to track my hours, invoice for my time, and collect payments. Start him there? I did work for my local YMCA laying out their marketing materials and big fall catalog of classes and services. Then bounced to a few small boutiques that needed logos and such. Hell local bands love freelance creative kids for their odd ball wacky designs. If they are serious about creating and being a designer, this is probably the best start, gives them freedom to succeed or mess up on their own and handle the consequences on their own.
Give them feedback as they ask for it. Then when they are naturally curious about how something is produced, and why a specific design is worse to produce than another, show them the way it's built, have them weed the vinyl for it, have them paint it, assemble it.