Welcome here, it is a worrying situation and I'm glad you came.
I guess I can almost understand your dad, I'm around the same age and have much the same sort of illnesses, though that was from being a policeman.
One can know what is acceptable, and your dad, when he came to visit regularly would have presented as ok. Maybe pride did that, maybe any one of a number of things.
Then there is "why bother", something that takes over when he is by himself and does not feel he has to put on the acceptable face. I had that for a while, though as I was living with a very caring partner things did not deteriorate around the house.
At the time I felt it was just me and as a result not worth doing anything not absolutely essential, and that snowballs, it can start out just not washing up and putting everything away into not washing, not doing laundry and not even purchasing enough food. There are other complications too, particularly if in pain with limited movements or strong meds.
Then it gets to the point where things are too far gone to fix, the mold, the floor, the laundry and so on. Then that leads to isolation as one does not want others to see the conditions inside.
I'm not sure of easy answers, it may be he has not been under effective treatment for this mental and physical conditions, or maybe just the physical ones. Proper medical ongoing mental support is needed. I still have it, and it turned my life around.
Getting your dad to seek this may be hard, and if you are not already familiar with it the Open Arms organisation (1800 011 046) may well be able to help, with information, counseling and referrals for individuals and their families
There are no doubt things that are extra important to your dad, everything from a worn knapsack to papers relating to claims, in-service matters and more. It might be hard to predict what is really significant
How you approach your dad - as you obviously need to talk I'm not sure, however I would suppose your aim is to get him expert medical support, and ensure the things he needs for pride, and security and are part of himself do not get tucked away during the move
I might suggest a respectful rather than scolding approach should be considered as pride may, like me, be integral to improvement.
Sometimes talking with peers can be a help, does he have any contacts with those he served?
Please let us know what you think, and do keep coming back to say how things are going