speech delay

21 months almost;Stephen-he has no speech whatsoever-he becomes frustrated because of this and on occasion has pretty violent tantrums, bangs his head on doors and the hardwood floors. I am a bit sad, confused and really aprehensive but at the same token want to be pro-active; if indeed he has a problem I want to nip it in the bud asap.I read on the computer that by this age 20-21 months our children should know at least 50-100 words if not more., he has zero!