Children develop at all different stages, there is a good chance that there is absolutely nothing wrong with him, speaking is just not his "thing". He is able to communicate with you, so he obviously understands enough.
My older boy was slower to speak than my younger. Having said that, he did know quite a lot of words, he just didn't string sentences until he was approaching 3 at that point, his 18 month old brother was holding conversations. In terms of ability at school etc. they are both bright boys, but the elder is still quiet and we still cant shut the younger up!
What I am trying to say here is that you are doing the right thing, keep chatting away without necessarily expecting him to join in and maybe it will just come and he will start speaking more.