PictureFit is a youtube channel that answers most fitness-related questions.
The guy is clearly a nerd who does his research, and he doesn't really try to sell anything.
Since your son is 14, these videos might be a good starting point.
Livestrong, Bodybuilding.com, and MensHealth also have tons of articles.
But Livestrong is sometimes more focused on the cardio/general health side of things. Bodybuilding.com is known for pushing certain supplements. MensHealth is pretty good, but it can be difficult to find exactly what you're looking for.