egg salad in a pita (wrap in plastic wrap before putting in baggie), string cheese (maybe he'll accept it because it's fun), frozen go-gurt (they thaw in time for lunch), grapes, apple slices, Lunchables chicken nuggets, goldfish crackers, Lunchables pizza, carrot sticks, raisins, cashews, almonds, fruit snacks (like Gushers), home made fruit leather (much healthier and easy if you have a food dehydrator), on the topic of food dehydrators you can also send banana chips, other fresh fruits like peach wedges or kiwi slices, nutella sandwiches (nutella is like peanut butter, but it's made with hazelnuts and cocoa instead) and so on... fresh fruits can really fill in the gaps and round out an otherwise thin meal.
Good luck!