I wish i had answers for you but can only make sugestions - obviousely you are grieving yourself -and maybe if you find ways of coping with your grief he will follow, but its really difficult if he refuses to seek help or talk about it - does he have any mates or another relation he is close too, you could alert to your concerns - perhaps he wont talk to you because he fears it will upset you more.
Not dealing with his diabetes is a worry and is probably adding too his mood swings along with the alcahol which as you know will just be adding to the sugar level chaos.
If you havnt already done so contact your GP he may be able to help get some bereavement counselling for you and if you feel stronger you will be able to cope better with him.
Tough love - there are degrees of that - from throw him out - obviousely not an option, to encouraging him pull his weight - at least physically -so can you engage him that way - painting his room, doing the garden if you have one, taking the rubbish out etc - obviousely cooking is one thing you probably need to be in charge of to ensure at least his diet is as healthy as it can be - but that doesnt mean he cant take part in the process, and things like washing his own clothes cleaning his room etc -
Doing everything for him might keep him sweet but he also needs to recognise he has a role to play in supporting you -maybe by giving the appearance of being the "strong one" you are undermining him, even though of course you dont mean too - but perhaps he needs you to lean on him sometimes, to give him the opportunity to step up in that role.
Financially even if he isnt working he is getting money for his alcahol - if its from benefits can you reduce what he has to spend on it - so ask him for a percentage of it for his keep ? food, utility bills etc - also dont keep any in the house -if you drink, tell him you know it increases depression and so you are not going to drink as you feel it making you worse (open the opportunity to have the discussion about your grief and maybe he will talk about his) - that way you are giving him a message but as though its you that has a problem with it - rather than that you are pointing a finger at him.
Finally - welcome
come back - use us as a place of sanctuary where you can write - because even the act of writing helps us deal with our emotions - and perhaps other here will have some sugestions or have been/are in a similar situation with a familly member.