I agree that you may want a second opinion. Does your son comprehend everyday requests like, "Go put in in the trash?" "Do you need a new diaper?" "Do you want a drink" "Do you want to go bye-bye?" My son is not 21 months and really didn't say much at 18 months, but could understand everything said to him. I didn't worry, as he was my 3rd boy and the first two were SO different in development that I knew every kid comes around in their own time.
At 21 months, my son now says "mom, daddy, drink, bye-bye, night-night, yes, Kiki" (his brother's nickname) and is adding new words daily.
My middle son, I recall wasn't saying even this much at TWO. At two he was still screaming when he needed something. When he turned three, he had an amazing vocabulary and was speaking in adult sentences.
I knew in my heart of hearts that my middle son wasn't developmentally delayed. The doctor tried to tell me that he may need a therapist, but the "Mommy" in me knew he was just on his own schedule. He'll be 4 in April and he's writing letters, sight reading words, getting his own cereal....
So when my 3rd son came along (all this after my 1st son was counting to 10 at 1 1/2 years old) I was patient and let him tell ME when he was ready.
And yes, you also have to consider the older siblings. My older two were always saying, "Baby wants this" "Baby wants to do that." and the baby would nod in agreement. He would "communicate" to his older brothers what he wanted and they would relay the message. Finally, before a doctor could tell me something was wrong with him, I started telling the older boys, "Let baby tell Mommy what he wants."