Kind of long.. not sure how to handle this

So I'm not quite sure how to handle this.. My oldest received a trillion dollar bill for his bday last year from his father. Last time I had them, he kept asking me if he could go to the store with it. I tried to explain the best way that I could that it wasn't real money.. He just got upset and wouldn't talk about it anymore. Again yesterday he asked if he could go to the store to spend his money. He basically won't take no for an answer, I don't want to potentially have him get embarrassed by trying to buy something with it. What's the best way to handle this situation? I really wish his father didn't tell him that the money was real, since he is a very gullible person like me