I was sick for many years with abdominal pain and had endoscopies, colonoscopies and blood work and told it was gastritis, and IBS by my gastro doctor who has been my doctor for over 20 years.
I became sick last March again with diarhea for 6 weeks. Had tests again and nothing found. It wasn't until I became jaundiced because the tumor was blocking the bile duct, had dark urine, itched and had clay colored stools that I had a CT scan. When my md who is also my friend got the results he was a shocked as me.
My advice to you is to stay on top of the doctors, try to get your dad the best care and have lots of hope. There are so many new treatments today for pc.
Good Luck and God Bless.