It will happen when you least expect it. My son is the same age and doesnt say much at all either. He points and blabbers and when I try to engage him in conversation he gets distracted with something else. That's boys for you. :)
I know he understands me b/c he'll get the sippy cup for me when I ask him and he sits down when I tell him its time to put on his shoes. When I tell him,"lets go bye bye" he immediately stops what he's doing and follows me to the garage door.
I started speaking at 9 months but my younger sister didn't say an understandable word until she was almost 2. As adults,here I am the boring stay at home Mom while she is the hot shot investment banker who graduated from Notre Dame with honors. ;)
So I wouldn't worry just yet. I have been told many times that girls start speaking sooner than boys. As long as he understands you,his speech will soon follow. But its not a gradual thing...it seems to just happen overnight. One day he wont have uttered an understandable word and you're worried that he's behind developmentak schedule and the next day he's jabbering out 3 or 4 words... just like that. Just like with walking. One day he was crawling around pulling himself up,and the next he was walking(almost running) around the room. I didn't even have time to prepare and oh how I long for the days when he was immobile. :)