I wouldn't worry. Is he veyr physical, met his physical milestones like crawling and walking early? Is he coordinated with his hands, has good eye/hand coordination? If so, then he is just using 'that side' of his brain more, and he will start talking soon. My oldest was the same, met all hsi physical milestones very early, was extremely coordinated and had, still has, the most amazing hand/eye coordination. He didn't so much as grunt at us until he was about 2 1/2, and then literally started speaking full sentences, and now at almost 5 he is very clear, extremely smart, and talks non-stop.
My youngest is 18 months and he started talking 2-3 word sentences around 10 months, but he is not nearly as physical as his brother, he is a klutz and not coordinated at all. He just uses the other side of his brain. :)
All kids develop differently, and I'm sure your son is doing well in other areas, and fine tuning other skills instead.