My son is 21 months old, and still no teeth. Is this really abnormal?

Probalbly not. By this age, your child should have about 8 teeth. Although there are vaiations in normal growth patterns, your child is really behind. You should see a pediatric dentist. A simple x ray would show if he is just behind or if he is missing one or more teeth. This is important since your son should be starting on solid foods already.
Probably. Usualyy a child has a number of teeth by now.Check with your pediatrician or a pediatric dentist for problems.It could still be normal varient.