21 mo old speech?

my son just turned 21 mo old and he has a very small vocabulary of intelligable words. He says thank you EVEY time he hands me something or if he wants something. other than that he says uh-oh occasionally if he drops something but doesnt address me as mama or my husband as daddy. he says the sounds but not really to me. sometimes if he is really upset he will say meme over and over. he has not digressed in speech he just never picked anything up. he understands several things though and I dont know if I should be worried. at his 18 mo checkup they said he scored really low in the speech dep and said to call and get him evaluated with a speech therapist. I am still waiting for her to have an opening. should I be worried or has anyone else seen this? btw he has excellent eye contact and I can tell his hearing is fine.