I have no idea what he’s doing because there are no details.
Training “3-6 days a week” is so vague it’s actually useless information. “Listening to his body” … about what? About training? His body doesn’t know anything about training, so he needs structure to eventually learn how he responds to training.
“Eating like a truck” means nothing because, again, it’s ridiculously vague. He should be eating 3 meals every single day. Most kids don’t do that, so if he does, that’s a good start.