I'm sorry, but I could not disagree with Leah more.
Yes, it's possible for some children just to catch up on their own, but that doesn't mean you ignore a problem that you know exists. We know much more about speech delays and how to help children past them than we did even 10 years ago, and I feel sure that if mothers and therapists knew then what we know today the mothers would have gotten their children assistance.
Anyway, back to you! At 20 months old I would not be freaking out that he was speech delayed, but because learning at this age is like building blocks - they need the basis of one thing before they can move onto the next- IMO it's irresponsible not to get a child who is delayed help.
My son was delayed and I got him evaluated through ChildFind. They came right to my home to evaluate him, and they came to my home from the therapies - all at no cost to me. He only needed therapy 2x per month,then 1x per month and by the time he "aged out" of the program when he turned three we ended therapy. His therapist thought he was all caught up anyway - mostly by what we were doing at home - so we didn't have him evaluated by the school system at that point (at age 3 if you still want services they must be evaluated by the school system and therapy becomes school-based).
A few months ago I went ahead and had him evaluated by the schools, just to be sure. He didn't qualify for any services. He's just started VPK (another great, free program) and is 100% on target in all areas.
So, I'd investigate ChildFind, and see how it goes. If you think your son needs more services than they are willing to give then you can consider private therapy in addition.
Another responder mentioned how her son threw tantrums when he wasn't being understood. My son was the same age yours is when I first contacted ChildFind on the recommendation of his pediatrician, who told me, "You don't want a 2 year old who isn't talking. Two years olds get very angry when they can't communicate or are misunderstood."
Best of luck!