T.H.
Hi there, my daughter was pretty slow talking at that age as well. At 18 months old she had about the same word count as your son does. However, I knew she knew what things were because if I asked her to get a shoe, for example, she'd retrieve it. I reported this to my pediatritian group and they said to wait a bit (they seem to be a more relaxed group than most), and by 22 months her language drastically improved within a couple of weeks. Then after that she spoke more and more. She's 3 years and 2 months old now, and her language capabilities are right on track. In fact, she used the word patient tonight in it's proper syntax and pronounced it perfectly. I am by no means urging you to second guess your pediatritians advice. I'm just sharing my experience. My opinion is that there is just too much pressure on young ones to perform that any perceived slowness is just jumped on. However, if your son seems to not know words or not understand most of what you're saying, that would be a different situation. I knew my daughter knew the words, she just didn't talk. I also used signing with her to get key needs expressed. It was really easy to teach. Just keep on talking to him, keep labeling things and it'll come around.
Hope it helps.