Because you are a US citizen, if the I-130 is properly filed before he turns 21 he is automatically protected from aging out as long as he files the relevant paperwork with the US consulate within a year after the I-130 is approved. They don't need to approve it before he turns 21 in order for him to maintain eligibility, and mentioning his aging out in the letter won't make a difference to their processing, as there is no reason for them to process it any quicker than anybody else's.
They normally will send you a receipt notice within 2-3 weeks after receiving the application, and that receipt is official acknowledgement that the I-130 been filed.
PD: Jan 2003 (EB3 rest of world)
I-485 Approved: July 2007
I am a layman, not a lawyer. What I write here is not official or professional legal advice. In addition, my answers on this forum are specific to the scenarios discussed in each thread and should not be generalized to other situations.