The OP is in Ireland. Ireland, England, Scotland and Wales are all technically considered "rabies free" so the chances of it being rabies is extremely, extremely slim.
It does sounds like something - a blow or other injury, might have caused neurological damage. But you also mention he has always had a bit of a screw loose - it might be he had something like a tumor that has been there for years unknown. There is really know way to know for sure without a postmortem.
No matter the cause, it is very scary, and sad, and I am so sorry you lost a friend. You and your BO will be in my thoughts.