Have you talked to the school? They may be able to help out, particularly if he’s already in the queue for a diagnosis. Maybe a note from your doctor would support this.
Help may be tuition support, guidance or even some extra time in the exam room - or doing the exam in a different area.
Anything you can do to stack the odds in your favor can help, and the worst they can day is ‘no’, so there’s nothing really to lose.
Finally, though exams are important, they are not the end of the world. They can be retaken later if necessary so try and remember to keep things in perspective and manage the stress accordingly.
Finally finally, good luck to you and your son.
Surely the simplest approach is not to use a wifi ssid that you don’t trust. Yes, mobile broadband is slower and more expensive, but is also more under your control, it is however harder for your boss to monitor.
If I was an it manager I’d probably want to keep an eye on WiFi traffic, even on the open ssid, just to see what’s going on. I don’t trust free WiFi.
If you can’t use 4/5g, and can’t find a suitable firewall or VPN, can you set up a different user on your phone? - and be disciplined in is use.