Preemie question!

Sharai

My baby was born at exactly 35 weeks on Feburary 13th. He will be 2 months on saturday and I was just wondering when will he start smiling? He has 2 cousins born a couple days after him (they were full term) who is already smiling and making cute little noises. I know my son is a little premature but should I be worried about it??? I understand it may take him a little longer to develope certain things. TIA