I have a passel of boys and a small/medium size backpack bag is a prized possession at my house. They put athletic shoes, a change of clothes, swimsuit & towel, frisbee & snack ... anytime they are going somewhere that they will be for awhile or may need a change of clothes. Should be easy enough to find a fabric with his interests, or a canvas and applique something on.
Something like the following link:
http://www.breakitdowndesign.com/bac...hapter_two.htm