My son is 22 months and has lots of words, many he's used for the last three months (at which point he only said "Ma", "Da", "That?" and "dog"), but most are the first syllable only and he doesn't seem to be progressing with the full pronunciation.
For example, he says "tah" for taxi, "pi" for pizza and "pan" for pancakes, and has done so for a few months, no matter how many times I say, "Yes, Daddy makes nice pizza," or, "Pancakes. Yes, we love them with blueberries," etc. etc.
He has one or two words that are more developed ("av-cah" for avocado and "out-sigh" for outside but these are exceptions).
He doesn't appear to have a hearing problem. Would you get his hearing checked, just in case, or wait until he turns two and mention it at his two-year-old check-up with the ped?
(Should add that I spent the afternoon with two kids his age, one of whom is bilingual and the other says sentences, such as, "I'm sitting on red cushion", so yes, I'm probably being a smidgen paranoid PFB mum.