I would say it's one of those three.
1. He had a part, but lost it (due to behavior, Carol, the full moon, whatever), so he "didn't even want it!"
2. He auditioned, was not cast, so "whatever, I never wanted the stupid part!"
3. He doesn't give a rat's hiney about acting but his mother is living out her dreams through him, thus "I don't want to be in the play!" as an earnest statement. He doesn't want to be in the play.
There's no real way to know for sure, but all are possibilities. Or maybe he was just being a kid, said the first thing that popped in his head to something the other kid said and had no real meaning behind it.