I remember having very similar worries about DS at this age - like you, I was comparing him to my NCT friends. People tell you not to but comparisons can be helpful.
In my case I remember freaking out a bit that we were massively behind because my friends DD was happily pointing to a frog, rabbit, baa baaing and moo mooing while DS … wasn’t! He didn’t even moo so you’re ahead of us! He only had about five words at that point.
I don’t know exactly when it started but I also know I found 16-18 months quite tricky behaviour wise, lots of tantrums, started being quite rough with other children (he had been a very gentle, chilled baby) so it came as a shock!
I know I was finding him a lot easier at the 19 month mark. He definitely had around 30 words at that point so somehow he must have acquired a lot in that period. By his second birthday (mid December) he had about seventy and was using sentences - in an extremely basic way - ‘bye bye daddy, hello baby, mummy here’ - but he is following a completely normal trajectory, although I know a lot of toddlers with a more sophisticated vocabulary!
The main thing you’ll be asked to check for as he approaches 18-24 months is understanding . It’s harder when they don’t talk as much as DS was never much of a pointer either, which always had me worried! He does point but on his terms 
if you said to him at 18 months ‘point to froggy’ in a book he wouldn’t. So I didn’t know whether he understood or not. He did and I realised once when my SIL told me that a toy ‘needs to be unscrewed at the bottom and then it works better’ and DS started unscrewing it!
Honestly, it’s easy to get hung up on these things. I did. I worried I’d done parenting all ‘wrong’ somehow. I remember that Easter buying so many books and endlessly pointing things out and that’s obviously not a bad thing but it took the enjoyment out of it. The main thing with books is they like hearing your voice, being close to you and it builds a positive association with books and reading. I wish you both well, I think you’ll be fine 
(We are now having this with colours - everything is green 
of course some of the wonder toddlers in my group know all the colours, but we’ll get there!)