Lots of good stuff posted above
I'm going to speak from my own experience. But also from others that I have sought out, or who have sought me out, regarding dreams of friends and relatives that have passed away.
Our family was once very big and I also had a lot of friends that have passed away. These dreams have started since puberty and I am in my early fifties now- and these dreams have not been surprising for decades.
That said there is, like most spiritual topics, a need for discernment. Is this a visitation or a comforting dream conjured up by the unconscious?
A few aspects I use that confirm the legitimacy of the visitation.
Joy, and or feeling the Holy Spirit.
To some degree these dreams are lucid. Your dad's ability to consciously think during the dream suggest it's valid.
What kind of film was this dream shot on? For me and a few others that I know, a visitation dream will look unlike others. It has a visually unique look .
We have all seen enough cowboy movies. Now the costumes, six shooters, and the buildings and the horses all look the same. But you can tell what decade it was filmed in.
...and I'm sorry but this is very difficult to explain
A friend or relative may often appear young and healthy and vibrant. Completely different than the person in the hospital bed or the casket.
Let's pick this up tomorrow
got to go cook some dinner. Paella.