It's difficult to offer thoughts without knowing your dad, or much about his condition. I think the best thing you can do for someone emotionally is to be a good and compassionate listener, without feeling that you need to solve everything. Tell him you love him, and that you're there to help in any way you can. When you look to far into the future, it can be scary for anyone even without cancer. I try to keep my dad focused on today, and having good days. Focus on what he does have and what he can control, rather than what he does not have.
My dad has been through a stage 2b non-small-cell lung cancer, and kicked it's rear end completely. Then this year, he was hit with a surprise new cancer-- this time stage 4 pancreatic/biliary.
Sometimes just something simple like going out for a ride together helps.