I think it is natural to look for other explanations Bee but I suspect that in your heart you know you are running out of time. Sit and talk about the past, say any of the things in your heart that you might sometime in the future regret never having said, remember funny stories from your childhood, take joy in any moments that dad is awake, hold his hand. If he is taking only sips of water, the home care team should be able to give you the sponge lollipops to dab on his lips and stop them cracking. His brain is stopping him drinking to ease his journey; as we dehydrate, natural pain inhibitors are triggered in the brain and sleep becomes longer and deeper. If he fancies a little ice-cream or yoghurt - cool things are soothing to a dry mouth - then it certainly won't do any harm and may be more welcome than trying to make him have things his brain is telling him he doesn't need.
I hope the nurses are correct and dad does rally but I would hate for you to be over-hopeful and miss the opportunity to say goodbye in the way you would wish. I have been where you are a number of times now and there are a few things that have happened with every single person I have loved and sat with; twiddling their fingers, plucking at the bed covers, seeing someone who isn't there. The district nurse once told me that the finger twiddling and cover-plucking are indicators that the brain is closing down.
Be brave; we are with you in spirit x
"Life can only be understood backwards; but it must be lived forwards." Soren Kierkegaard