E.S.
I felt the exact same way as you when my son was that age. I brought it up at his 1.5 yr appt and the doctor told me not worry, but it didn't help, I still worried. At 18 months, he could only say "ball", "dada" and "truck". But now at 23 months, his vocabulary is over 50 words. As the other moms have said, don't try to compare him to other kids and as long as you know he is able to hear normally, then try not worry. He'll begin talking before you know it and make up for lost time.