Smile frequency

Mi
Hi ladies! My baby will be 11w old this week, and I've noticed he's not a big smiler. He's fully capable of smiling, he does it every now and then. He smiles at his toys, at the fan, sometimes at me... But he's generally not into it! 
He was born 3 weeks early, so I know I'm supposed to take this into consideration for milestones like this. The pediatrician said as long as he has smiled, and is sleep smiling (he does that VERY often), he's just fine. I'm just wondering if anybody is in the same boat. Here's what I've tried so far:
- Exposing him to lots of smiling people, close up 
- Given him a crib mirror so he can examine his own expressions 
- Play baby TV shows for him with happy, singing kids
I'm beginning to think I just have a very stoic child! Could it really be that only the ceiling fan makes him happy?