What exactly does growth hormone do to a 17 year old boy, any negative side effects?

Depends: If a 17 year old is fully developed and has completed puberty, growth hormone will not be of any use. However if there is room to grow there may be some benefits according to some studies. There may be increased end height or they will just complete their growth faster. As for negative side effects, in general the hormone is well tolerated. Increased cancer risk is often discussed but not proven.