Boyfriend denied boarding by Delta due to a seizure from 2022 - looking for advice

I'm hoping someone has been through something similar or has advice. My boyfriend was supposed to fly home today on a Delta flight from Kansas City to Atlanta. He was able to purchase his ticket without any issues, but when he tried to check in online, he was told to see a Delta agent. At the airport, he was ultimately denied boarding. The only explanation we were given was that it was related to an incident from 2022. During that flight, he suffered a seizure onboard. At the time, he had not yet been diagnosed, but he has since been diagnosed with a seizure disorder and has been under the care of a neurologist ever since. The most frustrating part is that he was never notified of any travel restriction before today. He was able to book the flight, traveled to the airport, and then was told he could not fly. The agents at the airport were unable to explain how to resolve the issue or what steps needed to be taken. Customer service advised us to submit an email so a review team could evaluate his case. We have already submitted a formal request asking Delta to review his case, explain the reason for the denial, and let us know what documentation they need. We are also willing to provide documentation from his neurologist confirming his diagnosis and that he is medically fit to travel. Our biggest concern is that we have an international Delta trip to Greece in a few weeks. Has anyone gone through a similar situation with Delta?

If so: How long did the review process take? Were you able to have the restriction removed? Was medical documentation required? Is there anything else we should be doing besides waiting for the review team?

Any advice or shared experiences would be greatly appreciated. This has been incredibly stressful, and we're just trying to get this resolved as quickly as possible. Thank you.