our son was about that same age and i started to worry about him. he said "mama", "dada", "go", "dog", and "more"....and really nothing else. everything i read at that time said that an 18-month-old should have a vocabulary of 50-100 words. i was really worried.
BUT:
by 17 months he was talking a blue streak.
he is now 6 1/2 and has not shut up since! the kid talks more than anyone i have ever known. he does NOT SHUT UP. he has so much to say about everything!
so it sounds like your son is normal, at least in my experience. i would wait until he is 18 months before you really start to worry, especially if his receptive language is good--does he respond appropriately to things you say? if so, this is good receptive language and he is probably fine.