Definitely check out the
DAS threads, especially the first few posts with official and confirmed information.
You should definitely consider renting a wheelchair for him - just getting from the bus/tram drop off into Animal Kingdom (though the "Oasis, which is a long uphill trail with no attractions) is close to a mile. Walking around the World Showcase once, without going into any pavilions, is 1.25 miles. My grandmother has Alzheimer's, and I know motor skills and stamina can be affected. Even if he wants to walk, and you push the wheelchair empty, there may come a time when he is happy to sit down.
I would go into Guest Relations to ask about a DAS card for him - explain what you did here - that he gets confused, that being in crowded queues for long periods will overwhelm and upset him. I have discovered, at least in my family, that Alzheimer's is not terribly unlike autism in some ways, including autism-like meltdowns.
Even if you have the wheelchair, just don't bring it up. He will need to be present to get the DAS card, and they will take a picture of him. He does not need to be the one to do the talking, you can do that. You can also write down a letter to the CM explaining the situation, if he does not want to talk about it or listen to someone else talk to a stranger about it.
If there is a line in Guest Relations, he can wait to the side with family until you reach the front of the line, and then he can join you.
Hope you have a good trip. If he is excited about it, I would get a guidebook for him, or show videos on
Youtube of attractions, even if he has been before. He may or may not remember the past trips, depending on the day. We found with my grandmother, being very clear about things several times beforehand led to fewer problems later. Sometimes it helped for her to have written reminders of what was happening, things she could see over and over. Perhaps the hotel or flight reservations printed out for him - ask him to write his name on them so you know which one is his if you think it will help. That way, if he is confused later, you have a piece of paper with his name in his handwriting, showing he knew what was happening next.
Itineraries can be helpful, but plan very little, so you can take it easy on bad days.
Feel free to make use of First Aid in each park as a place he can sit quietly and decompress. It looks like a doctor's office to some extent, a place I am sure he has been before, so that will be a little familiar. Also, it is staffed by nurses, who will be able to help if you need it.
I hope you have a wonderful trip. Sorry about your father - it is a very hard thing to watch and feel.