Milly, I lived with my mum (in her eighties at that time) when my first marriage broke up. I intended to go back for six months and stayed six years. Like you it was a good arrangement - but we split living when she decided to go into sheltered housing (so the other way round from you) but she was desperately worried as to how I would manage! - it was the jolt I needed to get my independence again.
Your dad is actually younger that I am now, it is not old to find a little place of his own. The trouble is, you settle into a very comfortable rut, and he probably doesn't want to think about an empty house if you leave - but most parents would want their children to find a happy life independent of them.
Talk to him - you may be surprised at his reaction - he'll probably be upset at first but I hope he will come round. If he's not well you can always get him one of those Medic-Alert alarms - my mum had one, she found it a great comfort to know that someone would come if she fell or felt ill.