18 month old doesn't talk?

My son hit all of the milestones early, except talking. He is 18 months and only says "buh" for "bye". He seems to understand us though, I will ask "where is your cup" and he will go get it, and the same thing with his toy phone. He will go on and on in his language though. If you talk to him, he sounds like he is having a long conversation (just not in english). Everybody thinks his baby talk is so funny because he talks all day in it. He probably has 200 of his own words (not english), but never uses them for the same thing. So should I be concerned that he only has one partial word? He doesn't even say mama or dada yet..and when he does it is just part of his blabber.