I agree with PP about just having him buy an appetiser in non-buffet restaurants to fill up, but we found most of the serving sizes everywhere were large. Other than that, if you choose a lot of buffets & family style for dinner, which is not hard to do at all, I'd guess 1/2 of the Disney restaurants are buffets including some specialty ones like Boma. The family style bring out platters of food to the table & you can always ask for more of any of the dishes.
And then, if he makes the right choices at counter service, he'll get a ton of food. For example, at Flame Tree BBQ in AK, you can get a counter service lunch on the regular DDP that consists of a BBQ ribs (large serving), corn bread, baked beans, soft drink and a dessert (we chose min maid frozen lemonade for our desserts-it's like a 12 oz frozen drink...almost like a hard frozen sorbet) If we'd paid out of pocket, that meal would have been like $15.00...but it was a DDP counter service meal! If he "shops", he will be able to get that much food at about every park & resort food place.
We rarely could finish any meal, and we're not small people. And didn't need too many snacks between meals, either...full, full, full. We used most of our snack credits to buy breakfast pastry items (stayed at a resort with a coffee maker in the room.)