When does make believe become crazy?

My child attends a co-op preschool, which means we have to work in the classroom every few weeks. My son has become close friends with a particular girl, who for the purpose of this email we'll call Ana. As they have grown close, he has changed his name to Ana, insisting we all call him Ana. I was okay with this at home, but the other day I was working at the school overseeing the play doh table. There were five children sitting playing when my son came in a said "I'm Ana." All the kids at the table looked at me with eyes that said "what is he talking about?" After my son said "I am Ana" for the 10th time, one of the kids said "You are not Ana." This did not phase my son one bit. "Yes I AM Ana," he said and kept playing. He has since taken on other personalities. He takes on the name of characters in books, names of relatives after long visits, etc. but he always goes back to Ana. He's even brought my wife and I into the conversation. "Mama, you are Jasmine and I am Abu, papa is the Gynie." (Okay, I'll admit I did not mind being transformed into a skinny, Disney princess, but still...) I'm guessing this is a phase, but because the other children's reactions were that of "what is he talking about" or as we adults would say "he's crazy..." it did make me think about it....