According to this article, by the time your child is 18 months old, he/she should have a vocabulary of about 20 words, and a vocabulary of 50 words by the time he/she is 2.
THIS article from the Mayo Clinic says that, really, by the end of 18 months (so closer to 19 months, really) your child may only say 8-10 words and that this is considered normal as well.
It's also possible that your son is starting to use words, but you just don't understand them yet. It can take awhile for a child's speech to really catch up and be clear enough for even his parents to understand. Some letters are just classically difficult for children to pronounce so your son might be attempting to speak but is merely misunderstood because of his perfectly natural inability to make the "L" sound or the "th" sound.
This article on the what to expect website points out that children who are from bilingual homes tend to be late-talkers, but quickly catch up once they begin and become fluent in both languages. This same article seems to suggest that if your child can understand what is being spoken and respond to it, then expressive language is probably right around the corner.
Having said that, I can tell you that one of my nephews was a child who understood perfectly, but didn't speak. His parents got him involved with a specialist and his language skills progressed dramatically! He's 5 now and you would never guess that he'd ever needed to see a specialist.
I think, for now, I'd focus on what they tell all parents to focus on:
- No baby talk.
- Talk to your child all the time. Narrate your day, talk about what you're doing, etc.
- Encourage your child to say words and applaud attempts to say words even if there's a gross mispronunciation.
- Read to your child.
- Ask your child questions that require more than a yes or no answer. Like, "Henry, do you want the blue car or the red car?".
- Stretch the truth (from the What to Expect website): Repeat your child’s words and expand on them. If he replies “App-uh!” to your remark about slicing one, answer “Yes, this is an apple! We’re going to eat this shiny red apple for lunch.” Add those adjectives!
For the record, it sounds like your son is right on the cusp of talking, but the precursor to using words for communication is repeating those words. There's a lot that's involved in talking...muscles that have to coordinate to make the right sound, the understanding that objects have names, and the realization that using the right word can get you something. If he's still only repeating words by his next doctor's appointment, I would simply explicitly tell the doctor: He will repeat words that we say, but he isn't using words for the purpose of communication yet, then see what the doctor says.