Imitating Facial Expressions

Je

My LO is 7.5 months and doesn't really immitate my facial expressions. I try to sit him in front of me and repeat a facial expression to try to prompt him or do it in a mirror but usually he will just smile at me with a big grin. Maybe once or twice he has stuck his tongue back out at me but it isn't consistent and I'm not sure it was on purpose. Should I be concerned? Any other tips on how to encourage my LO to imitate me?