K.,
It is good that you are on top of it and being very observant--that way you can tell the doctor exactly what he is doing at this time. I wouldn't over worry yet--just be sure to tell his doctor all your concerns at his 12 month check up and ask him/her specifically what he/she expects.
I went through that with both my children. At 18 months, my daughter only had 2 words and the minimum should have been 7. I had her tested for free--ask your doctor about that. By the time someone from the agency called, set up an appointment, and came to my home to do the testing--about a month had gone by. During that time she increased her language usage to 20 or more words. Now at 4 1/2 you cannot get her to stop talking and she has an excellent vocabulary--ahead of a lot of other children her age. I do NOT shy away from big words either. I encourage her to try and pronounce them herself and we talk about their meaning.
My son just turned 2 --I had him tested around one b/c the doctor thought he might be developmentally behind physically also. The developmental specialists thought it was still too soon to tell. Between 16 months and 2 years he became a sponge and has been trying to say everything his big sister says--he even says "Thank you" when it is appropriate. My biggest concern now is --can other people understand what he is saying. I put him preschool for 1 day a week this year and his teachers think he's super smart.
All that is to say --keep being observant--start writing down the words he says clearly or at least sound very close to the real thing so you can give his doctor an accurate count. But I think it is too early to get worried.
You are probably already doing this---READ to him daily--he won't mind if it's the same book if it's one he likes. Talk to him during everything you do. I talk to my son at the changing table--I tell him everything that I'm doing. I also have a little music player that plays a different short tune
every time I hit the button and I sing to him. He loves twinkle twinkle little star--when I get to "star" he says it with me. I talk to him in the grocery store (I feel silly sometimes but most people don't think anything about it) I talk about what I'm buying or what color the produce is.
When I dress him I play hide and seek with his hand when I am trying to get his hand through the arm hole--I'll keep asking, "Where's your hand?" and when I get it through I'll say, "There's you hand." It gets very repetitve--but the next thing you know he'll being saying "hand." I talk about his hair when I brush his hair, I use the words sock and shoe when I'm dressing him. He just recently has started saying shirt and shorts and identifies them correctly when getting dressed--well, you get the idea.
Keep up the good work and stay on top of it. If there is a language problem--the earlier you catch it the better and you can get free or very inexpensive services up until they are about 3 years old I believe.
S.