I'm glad to hear that your family are pulling together and helping each other. Many times crises like this can help family members become much closer, and that's a blessing. As far as what to say or talk about, you two need to work that out among yourselves. A good start is to consider talking about things that concern/worry you both now, and also just things that interest you or you enjoy. See if you can find some things in common, of mutual interest. Also, you will want to ask or somehow find out how long the docs think he has to live. What you do and how much you change your life to help him will depend partly on the time frame. Its hard to believe, but some folks with advanced prostate cancer live for several years. For others, it may just be months or even weeks.
Pat K