Sorry for the late response, I just got back from vacation and felt like I really should respond to your request. I have a 27 month old with a very similar vocabulary that called me "Dada" up til just before his second birthday. He has an older brother that talks ALL the time, but he tries to talk, but just doesn't seem to get many intelligable words out at all!
I talked to his Dr. at his 2 year check up (which I went to 2 months late) and he too was concerned first about his hearing and then about speech. I am a teacher for Deaf and Hard of Hearing kids so in my heart I knew it wasn't hearing, but we had his hearing checked just to make sure, and it was normal. I was also slightly concerned about autisum (mostly because he WAS saying some words but then stopped saying them) so I went for a full evaluation at Child and Family Connections in Joliet, a state agency for early intervention for Will county. They had a team of 4 professionals evaluate him, including a Physical Therapist, Occupational Therapist, Speech and Language Therapist and Developmental Therapist, and found his LANGUAGE is somewhat delayed, however his speech is very delayed, mostly at the 12 month level. He is eligable for speech therapy, due to his delays. They also ruled out Autisum which was such a load off my back!
Before his evaluation I found a helpful websit that gives an idea of what children should be doing at different ages:
This is the American Speech-Language-Hearing Association website. They are the moitoring system for all speech and language therapists in IL. As a preschool teacher I found early intervention to be such an important thing when it comes to a child's success later on. I hope this information helps!