Disney deals with this every day. You'd be far better off planning for the best touring plan and options for your father than harassing guest services for months for something that will not happen. The best option for your father is to have a mobility device and to go to guest services when you arrive to try to obtain a DAS. He must have reasons why he cannot wait in a standard queue. No one can say whether or not he will be given one. A good touring plan and use of your 3 FP+ is your best bet - think of the DAS as a bonus IF you should get it.
May can be pretty warm, so take that into account. The heat will likely have a big impact on him. There will be other factors as well, ones that a DAS or all the fastpasses in the world couldn't help.
A nice Disney trip is definitely doable for your father, but only if you learn from your NYE experience and realize that people here CAN help if you are willing to listen. Disney deals with "my XXX is sick" all the time. It's not a special situation to them. IF you are willing to listen, the people on this forum can really help you plan a decent trip - with or without the DAS - but that requires you to be willing to learn from them.
May can be pretty warm, so take that into account. The heat will likely have a big impact on him. There will be other factors as well, ones that a DAS or all the fastpasses in the world couldn't help.
A nice Disney trip is definitely doable for your father, but only if you learn from your NYE experience and realize that people here CAN help if you are willing to listen. Disney deals with "my XXX is sick" all the time. It's not a special situation to them. IF you are willing to listen, the people on this forum can really help you plan a decent trip - with or without the DAS - but that requires you to be willing to learn from them.