He should get about 5-10 million, easily. That should set a person up for at least ten years. If he flops, he can go back to school or open up a business in time.
Just stating he can start a business is about as broad as you can get and guarantees nothing but the usual chance of failing when opening a business
If he starts school, I am sure many of the colleges would be impressed with his GED rather than an accredited HS diploma.
The money depends on whether he invests/saves it correctly but that isn't a guarantee
There is zero upside to leaving early. He will still be a top pick, he will still make a good signing bonus, and he will still have a bright future. We just want him to screw himself for our own greed of wanting him.