Rather than duct tape perhaps a pair of suspenders. Crossing them in the back would make it a little more difficult to manage and might give you time to assist him. Duct tape is a bit rough on the elderly skin so I would definitely only use this on the depends itself to make the harder to get off. Anything that makes the depends harder to pull down would be effective. Bless you and bless the sprinkled dog.
I do hope you find a solution and wish for you the patience and creativity to get through this phase of the disease.