Children vary in the ease and pace with which they acquire speech/language skills. But, having said this, your son has reached the point where this might be a concern. Consult with his pediatrician, with the objective of achieving a referral for a Speech/Language evaluation. You'll want to make sure that the oral-motor functioning is OK, and the speech/language specialist will be able to inform you about the developmental status of your son's development in this area.
My son is 20 mo old and saying Very little also. I suggest u have your son evaluated for possible speech delay- ( i am doing that this week)..If you go thru your states early intervention program...its free. Good luck