The term "retarded" is generally considered politically incorrect and highly derogatory in the twenty-first century, so describing Charlie as returning to being "retarded" is not a helpful description of his story. In answer to your question, yes - Charlie does loose the intellectual ability he gains; at the end of the story, he has regressed to a mental state that results in difficulties with communication, problems understanding and following directions, and other challenges. He does seem to retain some insights into why others act as they do and may have some understanding of what he had been capable of doing for that brief period of time, even if he can't describe or explain it any more.