Hi, I think just not doing a shape sorter in isolation is not cause for concern. he may not see the point of it or he may be more interested in other things.
My own DS, about whom I have no concerns, did not even engage with puzzles until after two. He has strengths and weaknesses and he also has things he is more and less interested in.
The shape sorter is (I think) testing cognitive (working it out) skills and fine motor/object manipulation skills. Are there other things your child does that display the same skills? I would suggest putting things inside each other demonstrates cognitive - for example, does your child try over and over to put a large ball into a small cup, or does he quickly work out what size thing will fit in what container? That means he is working stuff out.
If your son is able to select the right piece of the automobile cut out, get it right way up, in the right place, then I would say that is the fine motor/object manipulation, even if he has memorised it. If he had motor issues he wouldn't get the piece moved easily into position.
I am not an expert in any way but I was an overanxious mother who had a child with poor health, we were told he might have delay and I watched very intently.
At the end of the day, you know your child best - does what I say above seem ok or are you mre generally concerned? If you are more generally worried, then have a chat with nursery in more detail?
Sorry if I have gone on a bit 