My Cousin Thinks He Can Fly...

Hmm just take out all his costumes and burn them down seriously.
Also try to make him socialize with other people off his age.
When he realise that the other kids think he sucks he will realise flying is not cool and will stop to do childish stuff.

İf this wont work he has autism or w/e.

 

If this is true and since he has such a small attention span, I honestly think the easiest way to get away from the idea is to interest him in something else that will lure him away from the idea of flying.

 

jump of a plane with him