I know this from my son. Very active in sports, always a complicated eater but very body-conscious. I don't know how old your son is, but if he exercises every day, he's probably not that young. So from a certain age, we discussed what his body and also his head need to remain so efficient. We tried out different "tricks" together to add healthy calories to the day. And he had a thorough check-up at the doctor. He learned a lot about himself and his body, but also about nutrition. After all, nutrition is more than just calories and protein. As far as additional protein is concerned, he usually starts the day with muesli (often so liquid that he mixes it and drinks it). He adds unsweetened vegetable protein powder to it (he doesn't like sweet foods). He is always trying out new ones. He even really enjoys it. And since he has been actively involved in his diet himself, he eats more and better