Does your son get sick often? Is he lethargic?
If the answer to these is 'no', there is no problem- he's just thin. Some people are- and while they might envy the 'charles atlas' types as a youth, he certainly won't envy the bloated hulks they become in middle age. Chances are he'll outlive them too, and remain active and mobile to an older age.
Feeding him like a porker to fatten him up is ridiculous if there is no problem. If he's self conscious about being 'weedy', buy him some dumbbells. I was a 'weedy' kid in my early teens, and that's what I did, along with push-ups in the bathroom every night (I remember that because my dear mum thought I was wanking, what with the gasping sounds!). A few years later I was playing rugby with the navy.