He either needs treatment for his excessive and unhelpful drinking, or else he will not be able to do anything about the rest, if you ask me. Have you ever thought that he might be alcoholic or has that thought been pushed away?
As I interpret your situation, the diabetes is just secondary and it is really not a good combination, I agree, but I would pretty much tell him get treatment or else... You cannot talk to him when he is drunk, craving the next drink or just doesn't care because he is addicted physically or mentally to a drug that allows him to 'forget' and 'ah, when it doesn't work, I'll just have another drink'.
You need the good man sober first.
Alternatively, you need to set some very firm boundaries what you will accept in your house around your child, adult or not. Otherwise it will eat you up.
I do think though that the primary problem right here is not first and foremost his diabetes, and in any case, I don't think he will be able to get that under control until he has been sober for a while.
Sorry if I am out of line - I have seen couple family members drink themselves into massive amounts of trouble and leave this world early.
-M