Why I am unable to renew my AppleCare coverage on my sons 2019 MacBook Air?

Then there's another reason it's not eligible, but only Apple can tell you why.

You will need to contact Apple Support directly to sort this out then. Link-> contact Apple Support for Apple Care Issues - Apple.com

From this page:

Extend your AppleCare+ coverage - Apple Support

"Purchase the new coverage within 30 days after your original coverage ends."

If it the original coverage has not yet expired or it it is past 30 days since it has expired, it is not eligible.

My first reaction was: Apple does state that you may extend it. That means that Apple can decide whether to offer it or not. And, since they just introduce a new model, they might have decided not to support the older ones any longer. Just a guess.

Has the original Apple Care+ coverage expired on the MacBook Air?

If it has not expired you cannot extend it yet. You need to wait for it to expire.

If it has expired, has it been more than 30 days since it expired?

What happens when you try?

Are you following the steps here?: link-> Extend your AppleCare+ coverage - Apple Support