I'm going to ask my brother what my 78 yo dad's issue?
Do you think you know what is your dad's issue?
I am 98% sure, but I wanted to ask my brother just to be 100% positive.
It can be scary to ask someone right out what is wrong with them but it is better to do that rather than ask somebody else. They might not know the answer and it will hurt the person when they find out you were talking behind their back.
What should I do then?
You're a good child to worry about your father. Just go up to him and ask. Show that you're concerned and there to help.