There are 4 stages to pancreatic cancer.
Less than 20% of patients are able to have surgery and from what you describe your dad is not one of them, so the surgeon does not just want to cut.
It?s possible they are not telling you something, but it sounds pretty straightforward.
He could have anywhere from 4 months to 1 year. Maybe a little more, it depends on what kind of radiation he is getting and how well he feels and is able to perform daily living actives.