He rolls from his tummy to his back, every.single.time I put him on his belly. But, he has only rolled from his back to his tummy once, a few weeks ago and hasn't since.
And another thing on the sitting up - he can sort of sit and has some balance for a minute or two, but he often throws himself backwards. I talked to my friend whose a PT and works with preemies and she said that's because often times their backs are stronger than their stomach muscles so they do that. And that's also more common with reflux babies, which he has significant reflux and she said he isn't as comfortable when sitting so he will probably not want to and it will delay him that much further with sitting.
This is why it's hard for me to go by his age and his developmental stuff. Neither makes sense it seems. There's so many factors involved.