It's quite normal for kids to develop differently.
It seems your son is doing quite well, and I expect he will get more words very soon.
HERE IS A HELPFUL LINK FOR YOU ON MILESTONES FOR KIDS HIS AGE
As you can see, a few words at his age may be slightly behind the curve
but if this is the only issue it is not something to become overly concerned about
I agree with your pediatrician's advice to send him to nursery school
so that he can interact with other toddlers
and this may well prompt him to gain more words
and if he uses his current words appropriately and appears to understand more than he speaks this can only be considered normal
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If this problem continues after the age of 3 years do you think I should worry ?
Yes, I think that that would indicate a language delay and that your son might need speech therapy at that point, but it is very early to be 'borrowing trouble' and worrying about this.
Your pediatrician is an expert in child development. If he or she is not concerned you can relax and trust them. As shown in the above link, children are 'ahead' of other kids in some developmental tasks (walking for instance) and lag in other areas.
I think nursery school will be a very good thing for your son, and would not be at all surprised if his language skills develop very quickly indeed over the next few months.
You sound like a very good mother for your son. Try not to worry so much about this, but then again, being a father, I do understand your concern.