I know what you mean about how your dad can now remember your mum. It did take a bit of time to remember how vibrant my dad used to be, after seeing him so ill (this was 5 years ago).
I hope the time comes soon that you and he can remember your mum without saddness creeping in.
My mum was a worker and an energetic person and thankfully I am already remembering this more than her last few years.