It is true that people his age don't usually die of disease, but that still leaves other options than suicide. The most common for males 15-24, according to http://www.worldlifeexpectancy.com/usa-cause-of-death-by-age..., are traffic accidents, violence, suicide, poisoning "other injuries," and drowning, in that order.

Traffic accident would have been my first guess, being the most common death cause amongst the people I knew. I didn't think of suicide till I read the comments.

CNN Money reports that it was confirmed suicide.

It would say how he died if it were anything other than suicide.