Helping my son with healthy eating

My son is 14 and still growing pretty fast (1/2" last month). He's about 6' and around 135 lbs. He is extremely active --- he has at least 2 basketball practices a week that run 2 hours, lifts weights 2-3 times a week and trains on his own or plays ball with friends for at least 30 minutes every day, often more like 2 hours. His doctor has expressed some slight concern about his weight (he's right at the edge of underweight) and his coaches have expressed not exactly concern but at least a wish that he were heavier (he plays with kids who almost all outweigh him by about 40 pounds).

He lifts weights and would like to bulk, but I think he's a bit young (still growing, not yet adding muscle) and he doesn't eat enough to add weight. He also doesn't love to eat, and has some ADD, so he often just forgets to eat unless he's really hungry. If I make him food, he'll eat but if he has to do it himself, he'd as soon skip it.

I want to raise a healthy, responsible kid with healthy eating habits. I don't want to pressure him to eat because I worry that could set up an unhealthy dynamic. I also want him to take responsibility for his own eating. But... when I try that, he tends to not eat. He will skip breakfast, not eat his school lunch, eat a small dinner and realize when he's laying in bed at night that he's really hungry.

So my question for people with experience with this kind of thing is this: do I just keep reminding him to eat and making him food? Do I let him figure it out on his own (at least until he gets underweight)? How do I teach him to take responsibility for this himself?