I am very sorry to hear what your father is going through. My father is experiencing the same thing. He is 81 years old and had a whipple on May 31st. He has been so weak and slow to recover, that his oncologist refuses to begin any chemotherapy or radiation at this point and states he's at least a month out from being able to do that (assuming there are no setbacks). All the while, Dad's CA19-9 count continues to climb at a fast rate. (He had lymph node involvement, positive margins - at Stage III post surgery.)
My dad's surgeon told us that a whipple is difficult for a person in their 30s to recover from, so someone our fathers' age is in for a long, difficult recovery period.
I don't really have any words of encouragement other than to say you're not alone and that the good thing is that at least your Dad is able to receive some chemo at this point. That's truly a blessing!
You will be in my thoughts!