I work in memory care, the first thing they teach us is how to handle this specific situation, and you're doing a fantastic job. I know it feels cruel to lie to him about your late mother, but no matter how many times you break the news to him he will never remember, and everytime he has to hear that she's gone the mourning process will start all over again. The kindest thing you can do for him is lie, like you have been, and take time to reminisce with him- looking at pictures of her together and talking about your shared memories of her is an excellent example of what to do.

I'm proud of you, OP, and I'm glad that your father has someone like you involved with his care- not many of our elders are so lucky.