baby doesn't imitate

my son is 6 months old, and does not imitate facial expressions or sounds. he has been cooing since he was 2 months, and babbles mamamamama and sometimes a b or p sound. but i keep reading that even as newborns if you make faces they will imitate you (tongue out, etc.). the only thing he JUST started doing is shaking his head back and forth when grandpa does it--sometimes. and of course smiling in response to my smile since he was about a month old. thoughts? otherwise, he's super social--eye contact, smily, laughs, loves to play, etc. physically advanced--sitting up and has been for a while, etc. i can't help worrying about autism.