Yes he should have a lot more words by now. What does the Pedi say? I believe they should have roughly 5 words at 18 months and then at least 10 by 2 years.
I would definitely make an appointment to see someone about this.
My youngest brother (12+ years younger) was the same way and it turns out that he wasn't talking because his ears were full of fluid so he couldn't hear and repeat properly! He ended up needing tubes in his ears and as soon as the tubes were in and his ears drained he started talking quickly!(: