When my oldest was 18 months, I was worried about his speech, and mentioned it at his doc appt. The doc's advice was to play a little "game" with my son. When I handed him something, like juice, I would hold it just out of reach and say, "Juice. Juice. Can you say juice?" If he tries to say it, I'd give him the juice. If he didn't, I 'd keep saying, "Juice," watching carefully that I never got him very frustrated about getting his juice.
The main thing the doctor wanted to see was that he was hearing and at least trying to repeat words.
He also said that animal sounds count as words. So my son was roaring and barking and meowing and mooing, and they all counted.
Do you do sign language with him? Sometimes that can keep kids from speaking as early, but it's MORE than okay because they are and have been practicing a very important concept--that a specific symbol has a specific meaning. Signs are symbol for a meaning, and words are a symbol for a meaning. They also learn to put two or three signs together, which is the beginning of sentence structure. So people might think it holds their kids back, but really it is a jump start on language in general.
Has he had lots of ear infections? My BIL had a ton when he was little, and he didn't say his first "word" until he was three, because he basically grew up deaf. His first "word" was "uh" for truck. Keep in mind that he was THREE.
Another thing to keep in mind is that girls generally talk earlier than boys.
I didn't worry about my second son so much, but he really didn't seem like he was ever going to talk! He hardly said any words at 19 months. But now just try and stop him! He's 2 1/2, and he uses quite complex language. Just the other day, he said, "I love you past the moon, and on to all the planets!" What 2 year old talks like that? Everyone thinks he's older than he is.
My nephew was almost 2 before he said any words, but when he started, he spoke in complete sentences.
Kids take their own pace, and there's really nothing we can do to change that. Be patient and play that "game" with him. If you don't do any sign language with him, that will help, too.