He couldn't have been happier at the beginning because he always was being picked on and bullied. He was upset that everyone made fun of everything he did. He hated being the person that was always being bullied.

Charlie had said he was happy that he got to experience what it was like to be smart for once. He was always being bullied for a long time before he had gotten smart and he always wanted to be the one who people looked toward and he got his wish. So that is my argument about how he is happier at the end.