I kinda have the same thing with my dad and I'm living with him. He moans and moans. There is no point in asking how his day was, it's the same every time- negativity. The way I have dealt with it is to just be/ show kindness whatever because at the end of the day his griping is his own unhappiness with his life and not yours, my dad just can't express his feelings so it comes out in other ways. The kindness thing is working, he is much better. It just took a bit of time and hardening on my part in not taking things personally. We actually ended up having a nice evening watching telly tonight- not one single moan.
I guess what I'm trying to say is, hang in there cos he's your dad. But I know how frustrating it must be.
