Sorry that your son is in this difficult situation. If you get the services of an independent diabetologist, they might charge a fair bit to provide a report. If you get a report from your own diabetologist, they might charge a small fee for this service (but it is likely that they would not), they would have your old records, and they might be able to also supply a bit of a character reference.
Any idea how long he had been drinking and how much before his behaviour changed?
I hope all goes well.
best wishes