I'm so sorry for this sad time for you all. My dear dad has advanced alzheimers, so a slightly different scenario, but I so wish I could go back and tell him more just how much I love him, have always loved him, and what a wonderful father he has always been. I think these are things he will have known, but I'd love to know he knew them.
I would love to talk about all of the special times we shared together, wonderful memories, important things he taught me that had an impact on the person I became, and the things I am in turn passing on to my children.
And I guess, to tell him we are all going to be ok. But just how much he will be missed!
To a degree, take his lead, as there may be much he wants to say too, or he may just appreciate your company and comfort. Has he been able to discuss his wishes for his funeral? Forgive me, they may be stark words to read on screen, but sometimes it's an area couples find difficult to discuss with each other.
Reassurance for him that you will be there for your mum too.
Is there any way of reducing your work hours temporarily? Just to ease the pressure on yourself and be able to spend some precious time with your dad?