Generally protein is what fills people up.
You can google for lots of ideas but basically things like meat, fish, dairy, nuts, beans, eggs, oats.
So try and add something from a protein source at each meal or snack to help fill him up
Breakfast - weetabix fine if thats what he likes, but something like rice crispies won't fill for long. Add milk, and fruit. If he will eat things like eggs, peanut butter or greek yogurt are good protein sources.
Lunch - he has a packed lunch. Add things like cheese, or sliced meat into sandwich. or cubes or cheese, boiled egg, yogurt.
Snacks - nuts are good, you can mix the ones he likes with fruit he does ie cashews with some grapes. glass milk
Dinner - you say he's fairly fussy. start with an item he likes ie a chicken dipper, and add maybe mash potato made with plenty of milk or some cream. Gradually over time introducing different items alongside ones he likes. If he eats a fairly basic dinner, try adding a filling desert like rice pudding, yogurt, etc..
Have you tried getting him to help prep dinner? if he helps, he might try eating it ie homemade dippers? homemade fishfingers. adding things to flour, egg, then breadcrumbs can be fun (if a bit messy) for young children