On page 50 the S.E. Hinton writes, "Even his own family called him M&M. I tried to remember for a second what his real name was, but I couldn't think of it." When he moves into the "house" with the hippies, they call him "Baby Freak."
M&M is called M&M every where he goes. The only two other nicknames he goes by is "Baby Freak" and "The True Flower Child"