My daughter never liked peanut butter. She's 15 and still doesn't eat it.
If you need to send a sandwich, whats wrong with traditional lunchmeats or cheese? You could also use leftovers like chicken or roast beef. Not too many kids like eggs, but what about egg salad?
Why not forget the sandwich? Send a selection of healthy foods. Cubed meats (use leftovers or if you use deli meats ask them to cut the slices realy thick), cheese or even some peanuts. Maybe a hard boilded egg. Some veggie sticks or fruit. Some crackers. Yogurt is great.
If they don't refrigerate the lunches, buy one of those ice blocks to put in his lunch bag.