You are living with a volatile and unstable man but you are frightened of talking to a professional.
His GP is the first port of call.
Alcohol abuse will cause terrific mood swings - and he really should not be drinking more than the odd glass because of the high sugar content in ALL alcohol.
It is entirely possible that his lifestyle caused the diabetes, but it is not worth dwelling on. It is done and over with.
What is important is that his lifestyle today can make things much better - it is up to him.
I don't know what you are getting out of this relationship - but the only person who can really help him is himself. You can support him to a degree but it will be very tough for both of you.
Quite frankly I would run a mile. Not because of the diabetes - many diabetics live perfectly 'normal' and productive lifestyles. But because of the alcohol abuse. He clearly doesn't care about himself.