Totally normal Fruitloop

At 20 months you have a wide range of what is" normal" speech . If he is communicating in other ways eg. Pointing to something he wants and there are no other delays it is not out of the range at all. boys tend to be a little slower in speech than girls, for the most part. If at 2 years old he has not gained any new words or lost the words he already had, then that would be a red flag. 4 months is a long difference from 2 years old. In 4 months he will probably gain many more words.

Are you working at all with him to build his vocabulary? Flash picture cards perhaps? I worked together with a speech therapist when my son was 24 months and learned a lot of tools I practice on my younger daycare children to help build their vocabulary. My son was a slow on talker at that age. Now at 9 years old he is the most expressive, articulate child I have ever met !!