He (or his parents) must apply for his tourist visa - you cannot apply. His parents will have to provide written, notarized & verifiable permission for the minor to travel from Place to Place from Date to Date to stay with Relative Name, Address, phone, work phone, & cell phone to verify. Depending on age, the parents may have to pay the airlines for child attendant service.
Depending on age, general "street smarts" & sophistication of the minor, it is often unwise for unaccompanied minors to travel internationally. Unfortunately, there are those who prey upon minors when traveling, and a lot of scams, child trafficking, etc. Even fairly well educated, well behaved, reliable 16-17 y.o.s have been victims of predators. You and your brother & SIL want to do everything you can to ensure that child is safe!