Lots quit school for "real life", and for far worse reasons than having a substantial & profitable business, though I'd suggest there's a survivor bias to the ones you hear about - Branson, Gates etc.
Like I said, the only reason against is that you don't know what you don't know, and his parents will see real risks that he won't.
The time for an intervention was a while back, and it just doesn't seem like a terrible parental decision to throw your support behind a child with ambition and some success. (unless of course the business is propped up by parental subsidy, or favours from friends: free rent, broadband, meals etc. might have made a ramen-profitable business look like a fabulously comfortable one).
Who knows whether it's the right decision? It's a viable one while business is good. I hope his motivation stays strong for whatever comes next - whether that's expansion, a second business, college, or something else. Good luck to him.
I'd say to the kid: don't give up on school. There are lots of ways to continue formal education besides your current high school. Find something that works (see many ideas on this thread), part time if necessary, and do it. Yes, Bill Gates dropped out of Harvard, but he could return to Harvard if Microsoft failed.
> It's definitely a goofy, pompous, rambling letter written by someone who doesn't know what he doesn't know yet.
Here, you are quick to come to the conclusion that he is simply naieve. In that case, I too, may be naieve because I cannot quickly point out what it is that he is missing. Neither do you in your comment point it out. If you get around the language, which may seem pompous, it appears to me that this kid would actually like to know the answer to this very question that you have dodged.