I guess I'll add this to my own comment: working at a real job as a teenager isn't going to make or break his career. He's a smart and driven kid, just as you seem to have been. If he enjoys coding more than socializing or traveling or whatever, he should certainly code, but he's already well beyond his peers (and always will be).
It's just very different to work on your own stuff at your own pace vs. working for a boss. I also suspect completing more of his own projects would actually be better for his career than whatever a company would allow him to do.
Plus, that was what always impressed the most during job interviews. I was generally one of the top prospects in the interviews I did -- not because of my GPA, or the school I went to, or past job experience, but because of some of the interesting projects I'd done either on my own or to help out my father with his businesses. It shows not just skill (which lots of other people have, even if it might be a bit less than you) but ambition and drive (which, it seems, not many others have).
Which is exactly why I'd suggest working for someone if that's what he's into. Working with other engineers will give you a real sense of engineering standards, give you a sense of how to work on teams, help you set a reasonable pace etc. In a lot of ways at that stage in life your boss is much closer to a mentor/coach.
I would not suggest he does contract work, but instead finds a group of people who are interested in teaching and mentoring. I think we are comparing different outcomes in that sense.
Plus, with this drive and ambition his company might IPO when he's 18 and then he can retire and spend the rest of his life living the good life.
The real issue is that some people have a postiive attitude adn enjoy what they do, and some people live their life regretting everything they didn't do and not appreciating what they do.
Spend your summer doing something that you love. If that's coding, do that, and don't worry about getting paid for it yet. Also don't worry about building your resume -- you can start doing that when you're 14, if your heart is really set on it.
No one looks back on their life and thinks, "I wish I'd spent more summers working when I was a teenager." If they're lucky, instead they'll think, "I wish I could go back to those summers and really appreciate them for what they were: the last time I really didn't have to worry about anything."
For some perspective, I worked for software companies as a teenager and it hasn't helped me at all in my career. It wasn't worth the time, although having money was nice. But I don't remember the money now or what I did with it.