The only person who gets to plant any seeds of doubt with any authority is you and your DH! You know your child and it sounds as though he has great communication skills. i can't remember ages very well, it all starts to blend into itself after a year or two, but I do remember my stomach sinking when people updated facebook or told me about words that their kids could say. Then I learnt a couple of things...

1) The girl that had the most words in my mothers group NEVER SLEPT! Like Never! Of course she learnt to talk quicker. She was awake for so much longer and often in adult company, I decided sleep was more important than speedy speech development
2) Sometimes the only one's that can understand the kids wonderful vocab are the parents. And maybe DD wasn't as far behind them as I thought
3) Kids can only learn so many things at once. DD learnt to sing before she learnt to talk. She was singing Winkle winkly ittle ta, well before any sensible words and she also woudl much preder to learn to climb stairs. She has climbed to the top of the local park equipment since she was two. But now at 3, after a slow start, I can''t STOP her talking.

If your DS's communication wasn't so good, then you might have something to worry about, but if you're happy, don't listen to the other mums.
Also when bub arrives. DS might suddenly start using all the language he's been storing up to get his point across. DD's speech development accelarated when she no longer had our undivided attention and needed to speak up a bit to get her needs met.

Good luck
x