My son is also 22 months old, and talks about the same as yours. I seriously doubt your son has a speech delay. With my first son, it seemed like he didn't say much by 18 months, and I talked to his doctor about it. He said that usually, if parents sit down and think about it, and write them down, they usually find that their child uses more words than they realized. And I don't remember the actual number, but he said that by age 2, they should use at least 20 words. He told me about another test they ask parents to do to tell if it's a speech processing problem. Before you hand your son something, say, a cracker, hold it just out of reach and say, "Cracker. Cracker. Can you say cracker?" If he doesn't attempt any new words after 2-3 days of doing that with several different items (try toys and food), then they will be concerned.
My son says, "duh-doo" for thank you, and dinner, and lots of other two-syllable words. And up until 2 months ago, he hardly said anything. It's quite funny because, while he doesn't say many words, he knows about 23 of his letters. I don't think it's a problem with smarts!
I worked in our church's nursery, and there was a 3 year old who didn't talk much. I was wondering if I should be concerned, but his mom said that his doctor isn't, because he was the third child. First children get talked to a lot more than subsequent children, and often talk earlier, he said. And second and third children, if their siblings are old enough, often get their talking done for them.
I really wouldn't worry about it. Talk to your doctor about it at his 2 year check up and they can do screening there if they think there's a need.