think you need to check his percentile..
with regards to rollin, you can encourage him like when he is stuck, then help him roll over then praise him. slowly he will learn from there.
some kids r faster, some progress slightly slower, dont worry.
also, size shouldnt be an issue as long as he is growing..
pin has some classmates who r only a few mths different from her but yet they r much smaller in size (be it boy or girl)...