I would say that your boy is very much normal. Aside from the fact that he was a premmie - and they are always a little bit delayed in development - there are babies out there that are full term and are at the same stage as yours.

I wouldn't stress too much about it yet
FWIW - DS was talking HEAPS at 18 months - but DD (who is currently 14 months) is nowhere near where he was at the same age - so even siblings develop differently