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Reasons to worry about a 21 month old who is not talking:

Doesn't react to language when spoken to him, seems oblivious to what
language is for. Doesn't attempt to communicate with people in any way.
Doesn't make any sounds at all. Making no effort to attempt language.

Since none of those apply to your son, tell the commenters "Thank you for
your concern" except of course the child's dad who should be reassured. :)

Deborah in IL