This may not be the case, but:
If "when he does eat he eats a lot" means he is eating really huge amounts -- not just a big meal but constant and vast amounts all day, grazing constantly and taking in really large volumes -- he may be in a cycle of bingeing on food then refusing all food. Think about what he does when he DOES eat and if it is massive amounts, that's a red flag that he needs professional help. If it's just a healthy appetite (and frankly you may need to ask a doctor or nutritionist what that really should mean for a boy his age) it's not a binge-starve cycle but if he is binging and then refusing all food it's a cry for help.
I would say that would be a severe scenario and it's likelier that his metabolism is just one that is not the same as others' and he genuinely isn't hungry. But his body does need regular fuel. Take him for a checkup and insist that it includes blood work to check blood sugar levels, whether he's getting enough nutrients, etc. Yes, you will have to insist on that; they likely will not offer it as a matter of course, so be assertive. You just need to rule out any medical stuff.
Then ask for a referral to a pediatric nutritionist or registered dietitian who specializes in tweens and teens. He and you both need some sessions with this person to learn why he must get consistent fuel, not just "fuel when he feels like it." He also needs to learn why he doesn't feel hungry at times and how to read his body's signals better. Do not treat visits to the dietitian as punishments or as "You're so wrong about your eating that I'm going to prove it with this professional's help" because that will turn him off. Instead -- if he plays any sports, even solo ones, or if he wants to be healthy so he can do well in school, appeal to that. If he is in any sport with a coach, or if he participates in anything physical at all, have a coach or adult from that activity talk with him about consistent fueling of his body.
Do not make him sit at the table like a toddler until "it's all gone" and do not ground him. That will instantly turn his eating habits into a huge power struggle between you and you will lose. You cannot force another human being to eat and he knows already that he controls what he puts into his mouth, so telling him he must eat now, or must eat a certain food, will backfire. Appeal instead to his interest in his own health or his desire to play a certain sport or do a certain activity (which includes academics; body fuel is brain fuel too).
A friend went through something similar with her son and it required a nutritionist to work with them on what the whole family should be eating. It helped them to get that third party professional involved.