Yeah, I agree with the above, except to mention that it sounds like he's referring to the cast as a sympathy sheet. Usually kids sign each others' casts when they injure themselves, so it's just a funny way of talking about the cast.

Hi, There is no "sympathy sheet" normally in our culture but in this story I think this is his personal attempt to have people feel sorry for him because he is in a situation that he doesn't like. He humorously wants people to feel sorry for him so he can feel better about himself. In English we call this a "pity party". My guess is that no one will sign is paper and he will feel even worse!

I was literally looking at the book right now and I was think what it means so I searched it up and found this lol

his name for the cast