My 21 month old son got one of his front teeth knocked out, will this affect his speech?

It's possible. The front teeth are used when making s, f, and th, sounds. If it is only one tooth he should be fine at his age. By age 6 or 7 he will get his permanent front tooth an if he develops a lisp or other speech problem from losing his baby tooth early then he will have an easier time correcting it when the adult tooth comes in.
No. Not really, at 6 years of age he will begin to get his permanent incisors and these will affect speach much more. At his age one tooth missing will most likely have no impact on his speach.