Unfortunately, if he's like his dad, he'll just grow slowly until he's about 21.
Well, there's that. Pesky ol' genetics...
Perhaps a more general question would be, When a child's body size is extremely out of synch with his/her age, when is it appropriate to correct people?
I don't think it's ever in
appropriate to correct people as long as you do it simply and in good humor, without indicating that you're taking offense--"Actually, he's ten, so he does belong in the intermediate class", or whatever. It's not like they're making assumptions about whether he's adopted or something, and we seem to both be talking about people who he is interacting with on some level. (I wouldn't bother correcting a fellow passenger on the bus, but the librarian he may see again.) Adults with whom he interacts regularly certainly do need to know, as you say, so their expectations will be appropriate. (For example, depending on where you live, it might be appropriate for DS to spend a couple of hours at the library by himself, but it wouldn't be if he were the age he "looks". Vice versa for the 97th percentile kid--anyone who is supposed to be watching her needs to realize that she's not as mature as she looks.)
I actually think it would be ideal in your son's case if he learned to correct them himself--it would certainly be more effective. My brother was very good about it--if a waitress said what a pretty young lady he was, he'd just say, "Thanks, but I'm a guy."
If your son had been there without you, he could have said, "Sure, I've seen it before--I've had a library card here for five years now, since I was five. What I was actually wondering was.." and clarify his question.
It's unfortunate that children so often have to deal with the silliness of their elders (I am not a big fan of the 'talking down to kids' method of explanation, as it can be insulting even to a bright child of the actual younger age), but since, as you say, he may well be dealing with this for years, he might as well get good at it now.