Fat - sometimes a LOT of fat. Like starting the day with a cup of cream whipped up and flavored with vanilla and dipping strawberries in that. Eating a quarter pound of fatty salami or a wedge of brie. Dipping bread in olive oil and balsamic. I’m not saying to try to get your kid into ketosis, just let him eat as much fat as he likes, and sometimes this may be a lot. The examples above are all favorites of my kids. They don’t want them every day, but when they do ask they mean it. This is hugely calming for the more intense kids, and I know from accidental experience that a low-fat diet leaves me edgy after a couple of days.
Protein - Essential for making enzymes and neurotransmitters (aka brain chemicals, though they aren’t just in the brain). Count grams for a few days to see where you are. Baseline goal is half your weight in grams each day, but growing kids and athletes can need more. It’s easy to end up low on protein with cereal, pb and j, pizza when life gets busy.
Fiber - Far more serotonin is made in the gut, for the gut, than in the brain. Having enough fiber promotes a good bacteria mix and lining for the intestines, even blood sugar, regular bowel function, and a balanced immune sy
stem, all of which help to calm the nervous system. Any whole plant food has some kind of fiber. Go for variety and use the winners.