Spread mayonnaise and mustard on each of the bagel halves.
Top the mayonnaise side with lettuce, tomato, chicken, cheese, and close the sandwich with the other bagel half.
Pack it for school:
Slice the bagel sandwich in half and pack it inside a lunch container. Add healthy sides, like fresh fruit and veggies, to his lunch in a separate compartment or containers. Refrigerate until ready to pack inside a lunch bag, with or without an ice pack.