Agreed on above – just say you are him. I do this with a couple of our household accounts that are in my wife’s name (Virgin Media in particular, any time I have to report the service as down) – its kind of humorous when they ask me “are you Mrs Krixmeister” and I just answer “yes I am”. You’ll be fine, nothing at all wrong with it in the grand scheme of things.
Downside of being honest and explaining the situation is that you may (will?) get a minimum wage phone agent who’s job isn’t worth bending the rules a bit.
And finally, best wishes for you and your family during this difficult time. Do get as much time with your father while you can. I lost my father about 18 months ago, not a day goes by I don’t think about him.