I would not worry at all! My brother didn't speak a word until he was 3, and in grade school, he tested at the genius IQ level. My cousin didn't speak until she was three, and then she busted out with complete sentences out of the blue. When my daughter was 18 months, she had only a few words and my ped wanted to get her hearing tested. It was fine. At 21 months, my DD still only had a handful of "words," just like your son. She pointed to things she wanted, she made happy or not-happy sounds, she used some sign words I had taught her, but she still wasn't even saying mama or dada. I knew she was fine. She didn't show any other signs of autism or any other developmental delay. In fact, she was off the charts as far as motor skills went. Our ped told us she was very concerned and we should have my DD evaluated for a learning disability right away. I said no. I wanted to wait a little longer because in my gut I just knew she was fine. My ped referred my DD to the Early Start program, who called the house and insisted that I was doing my child a disservice by not having them come to the house to "evaluate her learning environment" and getting her in therapy. Therapy? Just because she isn't speaking the "required" number of words yet? We still refused, but were now scared by this time. A good family friend, who has a masters in childhood development and works with special-needs kids all the time, came to the house and did her own evaluation. She said that kids who are progressing with or ahead of the curve on motor skills often don't feel the need to speak because they can communicate their needs and entertain themselves by using their body. She said DD was just fine and told us to just talk as much as we can to her, try to engage her in conversation by using lots of eye contact and questions, and acknowledge and reward any attempts to talk, even if it's "gibberish." For example, I started requiring DD to say thank you every time she got something she wanted. Of course, it came out "DA!" but as long as she said SOMETHING, I accepted it. I didn't make her say it over and over and over again until it sounded like thank you. Sure enough, shortly after her second birthday, DD said dada, then mama. Each day brought a stream of new words, and about a month after her second birthday, she was speaking in two- and three-word sentences. By 2 and a half, she was using more complex sentences, and by three she was caught up to her peers. AND, she talks nonstop!
Do not worry too much, and stop and really, really ask yourself, honestly, if you think something is wrong. Does he communicate his needs? Is he affectionate? Does he make eye contact? Does he engage you and other loved ones? Is he a happy kid? If yes to these, then just let him be. Have his hearing tested to give you some reassurance ( I did because my brother did have some hearing problems and I was worried it might be genetic), but if that's fine, just read to him, talk to him and give him time.