@Elizabeth1827 I'm not sure about whether there's a widespread messaging problem, but you might suggest to him that he use a different device or a different browser. If he's using the phone app, suggest he use a computer or vice versa. Or maybe he's not looking in the right place for messages? Is he an experienced Airbnb guest? (i.e. would he be aware of where to access his Inbox?) Also tell him to make sure his notifications are turned on, so he gets a notification when he has a message.
Are the messages showing as sent on your end, or have they disappeared?
You can try to get Airbnb help, but I know it's frustratingly slow right now. So if time is of the essence, you could ask him to text you his email address and you can end him the check-in info by email, if messaging isn't working.
As far as the extra day, if it proves impossible or too late to change the dates, if it's just an extra day, and he seems like an honest, reasonable guest, you could just have him pay you for the extra day outside the platform. I wouldn't normally suggest that, but I've done it myself once when the platform was being glitchy. Was way easier than me or the guest wasting a lot of frustrating time.