Hi R.
My daughter is 18 months old and says maybe three words reliably - mama, dada, and coco (one of our dogs). When I asked the pediatrician if that was normal, he said absolutely, and that by the time she was two I could expect probably another few words reliably. What I mean by "reliably" is, she will generally say something once or twice, and then I won't hear it again for a while(ie., cup. juice, yellow, etc), but reliable words would be those I can count hearing.
In my opinion, and actually in her pediatrician's opinion as well, what is most important at this point is that a child is understanding things - my daughter knows colors, names of objects, and can follow directions (ie., go to the fridge and bring me your bottle) by pointing and doing, but she can't say them yet. If this was still going on a year from now (at 2 1/2), I would be concerned. But, you know the story goes that Albert Einstein never spoke other than a word or two until he was 4 years old!
Talk to your son's doctor at his next visit, I guess, but it seems from what I have heard that it's not an issue at this point.