• I eat 2/3 of my kcal at dinner almost every day, it's totally legitimate if he can adjust to it. Assuming dinner is nutritious he should absolutely join you--IME civilized meal culture is the #1 way to fix health.
• the damage of cereal isn't really "26 carbs", it's harsh grain allergens, added sugar, and mystery flavorings. If he needs AM carbs maybe suggest plain puffed rice or oats with toasted coconut chips and berries? Lots of ways to capture the experience without refined sugar and wheat, though you mentioned he's hungry between meals so protein would be better.
In the end it's his body. You can share knowledge but you can't make him care.
Lots of: urban hiking, cycling, sprinting
Lots of: fresh meat, seafood, eggs, organs, tubers, starch fruits, vegetables, meat fat, dairy fat, oil fruits
Some: cured meat, dairy protein, sweet fruits, rice, pulses, tree nuts, oil seeds
Minimal: soy, refined proteins, sugar, liquid carbohydrate, grains, refined oils, peanuts