My ped has always said they should have five words by 18 months, including mom & dad, animal sounds, anything that is oral language representative of something.
At 21 months now, I'm assuming the Dec is the 2-yr check up, right? I would say only 5 words at that point would be concerning (as it seems your Dr said?) and so you all should check his hearing thoroughly and make early intervention appts for evaluations.
I would try not to worry about it until them - my son was very similar, just a few words at 18 months but by 2 yrs he was exploding with words. They all came all at once
