He's 20 not 10 so explaining the rules to him should go a little better.
You can also help by "doing the math" for him and giving him some flexibility. For example:
If he's eating at a QSR that has fountain drinks where he can get refills make sure he knows to grab that drink and not a bottle or additional bottle such that he uses a snack credit and his QS credit.
Make sure he knows he gets dessert with a QS meal, he can choose a combo meal to fill him up as an entree and dessert to top it off or to pocket to have in an hour or so since he's a pit.
Suggest he tuck something from your kitchen stash to carry into the park, something he can munch on while standing in line, etc. Something that won't melt.
He should probably always eat breakfast in the room, the QS meals aren't likely to carry him until lunch and he'll use another QS credit at lunch. Something with protein will last longer than something carb focus (eggs, breakfast meats vs bowls of cereal only).
Do your part too, if you select places for the family to eat together put some buffets and AYCE on the list so he can indeed eat everything that isn't nailed down.
Finally, if he can't follow your rules. Get his park ticket put on a card with no entitlements and then he'll have to come to you for every morsel he eats out or he'll spend his own money or he'll eat in the room. If you're really concerned then just do this from the start, only let him have a park ticket. No entitlements or charging allowed on the card.
Good luck.
