So much good advice on here!
Make sure all bills are in your name - it just makes things easier. Don't forget to notify the council afterwards so the council tax is changed. Also his dentist/doctor etc. His doctor should know, but in a busy practice it can get missed.
Put all the relevant paperwork in one logical place, where you can find it. Bills, life insurance, pensions, house deeds if you have them etc.
I acted as executor for my mother and I found that while the 'tell us once' service was very good, it didn't actually do everything.
Discuss his preferences for his funeral etc. When my MIL got her terminal diagnosis the first thing she did was to go straight to the Co-Op funeral parlour and arrange and pay for her own funeral. It made things so much easier for those who were left behind.
His will will state what is to happen to the big things.... but he will have little things he might like to give to specific people. Encourage him to make a list and decide what small bequests he would like to give. When I got my cancer diagnosis that was one of the first things I did (I should still have a few years left so I may need to update it). He may have a favourite pair of cuff links he would like to go to somebody, or some sports memorabilia.
Encourage him to get in touch with people he may not have seen for a while - if he wants to. One of the jobs I did when my mother died was to contact everyone in her address book to let them know. Her nearest and dearest knew anyway, and we made sure we telephoned those with the details of the funeral, but there were lots of other people to contact. We put a funeral notice into the Times, the Te!egraph and the Western Morning News, as we knew those were the newspapers most of her friends read, but I know that's probably not very relevant now. I ended up buying a stash of suitably sombre small notecards (so that I didn't have to write too much) and several books of stamps so that I could let people know. You may we!l have a list of email addresses you can use. I only had one, for relatives in New Zealand. Despite doing that there were still several people that were missed and it was very upsetting to pick up Christmas cards that had been delivered to her house.
Talking about Christmas, or indeed any other celebratory time of year, maybe start thinking about how you are going to manage that once you are on your own.
We intend to go to our local funeral director and put plans in place in advance. We should have enough savings to pay for our funerals, but it would be helpful for our families if they didn't have to worry about that as well. If I am lucky I will have another 10 years maybe, but realistically it could be 2 or 3, so sensible to do that soonish.
Think about the wake - does he want one? Most family and friends will expect one and people like to get together to reminisce. Is that something you would do at home? Would you do it in a hotel or restaurant? Maybe you could look into what is available and choose something together.
Does he want a church funeral? Burial or cremation? Do his family life close by, or if they are a long way away (abroad) would it be wise to let them know the situation so they can think about how they might attend when the time comes, if they can. When my aunt died last year the service was live-streamed from the church so that remote family could attend virtually.
I'm sorry, this is awful for you, but maybe being able to talk it over together beforehand will help you to cope afterwards. Everyone is different - I am autistic so it is important for me to plan ahead as much as I can. I wish you both the very best.