I don't think there is some new insight in his letter, though I agree the message is clear, and his decision made. But lots of 15 year olds think school is boring and pointless, or think they can do anything based on the tiniest success.
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.