Some of the most common causes of low testosterone in young people are exposure to environmental estrogen's in plastics, household cleaners, prefumes, paints, solvents, exhaust fumes, and other chemicals.

I'd also look at diet as well.  

A low fat diet will reduce testosterone levels in men.  Make sure he is getting plenty of fat from sources such as salmon, almonds, egg yolks, and clean meat sources.

Also, if he's eating most of his calories in the form of junk carbohydrates such as soda, cookies, chips, crackers, cereal, etc.. that will have a negative impact on testosterone levels as well.

Finally, make sure he is getting plenty of sunlight exposure. Lack of sunlight will cause vitamin D levels to drop, which will decrease he testosterone levels as well.

You can read about many more causes of low testosterone by following the link below..

http://www.boost-your-low-testosterone.com/cause-of-low-testosterone.html