DSS#1 is handicapped and considered severe by most. But at 12 mos he had lots of words and by 18 mos probably close to 50. Guess what, at 22 YEARS old he hasn't progressed beyomd 18-22 mos. and that is with 22 years of speech therapy, some years intense. He was my early talker
DS#2 had hardly a word when it started to emerge at age 2, and it wasn't because his brother talked over him.

At age 4 the preschool's annual speech eval by ST wrote a report that he was behind, had sound issues and needed therapy immediately to correct asap. I said "no thanks, he is just 4 trying to use big words". At 17 he doesn't shut up, has no speech impairment and is perfectly fine.
DD#1 decided to begin speaking very young and used huge words. But many she pronounced wrong. I had someone look at her and they said she has vocab way beyond her sound capability. She'll be fine. At age 13 she is just fine and also a continual talker.
I've been involved with multiple therapies/therapists for 22 years and no way at age 18 mos would I worry yet. Here is something to try. Our many speech therapists we have had all said it is much easier to sing than talk. I know my DS can sing very well. See if you can get him to sing along with some songs. And if he is responding and understanding all you say, I would just take a wait and see approach.
Don't get freaked out !