real hope now is whittling away at your resolve until you change the grade to shut him up. I think it shows that fundamentally he is unwilling & scared to run this up the chain, and despite his seeming lack of awareness, I think he gets that your accommodations have already been what any higher-ups would deem to be generous if it comes down to a open discussion between all parties.

The books are not opened to account for mysteries of emails that end up in the Bermuda Triangle.

Or maybe he really is dumb enough to take it up with management.

Given your description that he is the better writer of the two students, he might feel confident enough in his ability to make a shoddy case sound solid, but I doubt it.