Toddler talking help!

Anna
My toddler is almost 20 months old and he really lacking in the speech area. He is smart as a whip but when it comes to talking he knows about 4-5 words. 
Bye 
Ma 
Da
Baba 
Kitty 
Sometimes he will say two, go, rock. But those have only been said a couple times. 
I'm starting to get worried because other children I know at the same age are speaking 50+ words. 
Do we have any other moms with kids that were late bloomers in the speech area? 
I'm hoping he's just absorbing all the words we try and teach him and one day he will just blurt out knowing so many, but who knows.