Don't diagnose him, that's the worst you can do. (I know this has already been said, but...)
Manage to get him see a psychiatrist.
He might have lots of different things. Usually the only non-psychiatrist able to "diagnose" DID are the patients...
He could have Alzheimer's or any other disease which involves memory loss. It could be everything. A brain tumor, Hungtington's... But neither we nor you are qualified to diagnose him - so if you want to help him, BRING HIM TO A DOCTOR!
Cheers and safe hugs if wanted!
Lana
Lifelies