Don't ask him to make a bucket list as that's depressing and could lead to disappointment. But it there are things you know he always wanted to do that are actually possible, now might be a good time, so you can make some good new memories. (I'm thinking about things like join the National Trust or get a garden makeover...)
In the background, there's a lot of stuff you could do, such as researching dementia, making yourself aware of care homes and day care centres in your area, separating your bank accounts, paying for a funeral plan (sorry I know that's depressing), applying for Attendance Allowance (not means tested) and Council Tax reduction, looking into all the rules around funding care, etc.
Also, try to think about the future - your own future, as you are part of this 'journey' too. Have a think about what your lines in the sand might be.
And you've already done the most important, useful thing, which is to join TP!