Free: Contests & Raffles.
Ground blind and let the kid play a video game or something to keep him from getting bored.
I will probably get blasted for this but here's what I would do with a kid that gets bored so that he has an interesting and fun experience. I would take him road hunting and do a few short hikes and glass over some areas in between the road hunting. Break up your hunting methods rather than sitting in a blind all day, hiking all day, driving, or glassing all day. Break it up and do a lot of different stuff, the attention span for hunting will likely be longer.