This is an absolute no-no for a player or parent.
Actually, there is no place for parents EVER to be engaged in these conversations. So I appreciate that the OP posited this in the context of e-mails the player would be writing. The rest of my comments pertain only to player communications, because we already know that the parents aren't supposed to be speaking to coaches at all (unless and until it comes down to talking money).
As to player communications, here's the reality: Your son is in the class of 2021. He has to compete against players in that class, and beat them out, if he wants to get recruited to any particular school. I don't care if he's 12 or 17, his class dictates who he has to beat out. If you think he's at a disadvantage, you can consider reclassifying him, but as long as he's a 2021, that's all that matters as far as he is to be concerned.
Whenever a player (or parent) mentions this, it comes across like "loser's limp," i.e., an excuse for not performing up to par. Thus, you should never, ever do this, as it makes you sound like excuse makers, and it carries the implication that the player pretty much never performs on par vis a vis his classmates.
Also, the tendency to do this is a holdover from youth ball (pre-HS travel teams) and is a sign that there may be a lack of maturity or a failure to appreciate that you're in a different situation now. Meaning, if you want to be treated like someone who could survive and thrive in a collegiate environment, act like it.
If there is a time and place for someone to discuss the extent to which your son is projectable, then his HS or travel coaches may want to raise his age within the context of whether or not he may have a lot of growth left in him. However, this is often more a function of things like how big his parents are, the age at which his siblings topped out, etc., than when his birthday falls. There are kids who peak early and kids who peak late. But the key here is, a third party (the HS or travel coach) is in a position to speak about such things without blow back. Your son is not.
These are the reasons, but the rule is simple. Your son NEVER mentions his age.
You seem to think your son's late August birthday is unusual. It is not. Probably 20-30% of those competing against him in his class were born not more than a month earlier than he was. Moreover, many of his opponents will actually have some 2022's on the roster. These are other reasons why focusing on his age is counterproductive. The only relevant question is whether he can compete with the other guys on the field ... or not.