First of all, every child is different! The fact that your son understands you and is able to communicate his wishes shows that he is just fine! I think that is perfectly normal to have 4-5 words. What I found with my two kids, is that between one year and 18-24 months was the most amazing growth for them. My son did have a speech delay that we found at about 2 1/2 years- his vocabulary tested at a 5 year old level, but his articulation was at about a 2 year old.
I also found that my daugther's vocabulary developed much faster than my son's. She has been talking full sentences since she was about 18 months. Whether that is from being a girl or the second child, I am not sure-
On the other hand, I have a niece that just started talking and is still only saying 3 or 4 word chuncks, and she will be 3 in January. EVERY CHILD IS DIFFERENT!
If you have real concerns, feel free to call your pediatrician. They know all of the milestones that your child should be completing and when there might be a problem.
Good luck and enjoy the vocabulary boom that is about to happen (Oh yeah- from previous experience, be careful what you say around them... they are great at repeating private info!)