Hi, Kitty! I am a great believer in dialogue and I am glad you took the initiative to make it happen. It is an illusion that we can protect our loved ones from pain if we don't reveal our own info and emotions. They are hurt anyway and sometimes more deeply when it is discovered things were kept from them. My wife, after 50 years, still has emotions about her mother shielding her from her dad's illness and death. Of course, her mother thought it was best at the time.
I would begin the dialogue, not with a barrage of questions, but inviting your dad to express his feelings, his kowledge, etc. and just let him talk for a while. Have questions written down, and ask them after he has had his say. As you have already shown, continue to show your love, caring and support, with your words and your actions.
God's grace to you all. Don