Eat. That's as simple as it gets... he wants to put on weight. He needs to be in a calorie surplus. Probably has a high metabolism like my little brother, he is 13 y/o weighs 118 lbs (almost no fat on him from an untrained eye, tiny pinches is it)... just got him working out and eating more, he's strong as hell, benched his own weight first session... for 6 reps, I had to steady the bar but the kid reped it out.
Now if he wants to be healthy and gain weight, eat alot of healthy foods. It's harder than going to McDonald's a few times a day, but much healthier for the youngster in the long run.