Both my sons started doing regular workouts at the gym in their teen years (not sure how old your son is) and when they started they were just typical young boys. According to them the key is too do hard aerobic type exercise (or running) and then combine that workout with lifting weights. 3-4 times a week or more. The lifting of the weights has bulked both of them up, which is exactly what they wanted. It took 3 to 4 months of working with weights before they each saw significant results. Now they regularly play tennis/volleyball and run the park in addition to the weights/gym. Tell him to keep doing workouts--he will see results if he sticks with it. Good Luck!