Mine did the same thing for a while. Stop responding to his pointing at objects and tell him you don't know what he wants, that he needs to use his words. That's what got mine talking at 18 months. I was SO frustrated with his pointing and grunting - I almost yelled at him and said "Whatever it is that you want, you can have, but just say the word and stop that grunting!!!" Clear as a bell, the kid said "Cookie"! And that was the start.
Like you, I had checked with the pediatrician and she said we'd schedule some tests if he wasn't talking by 18 months. But she said most kids with significant problems are delayed in more than one area.
During all that time when my son wasn't talking, his brain was developing in other ways. Your child is probably way ahead in other areas as well. Do you do play groups or story time at the library, or other situations where he will be with other kids who are talking? He may not be talking to you because he doesn't need to - you anticipate his every need, you know his signals. How about some musical tapes with fun, repetitive songs? I got one for my son that had his name in it - all kinds of things about taking a bath, playing, going to bed, etc., but they all said his first name in the lyrics. The musicality made it more "interesting" than conversation too. The repetition and personalization encourage both of you to sing along. It might help, couldn't hurt.