Ultrasound or CT scan?

My dad was told many years ago that the food stays in his stomach for 24 hours (?) instead of being digested at a normal pace. He wasn't told what to do about it, though. Nowadays, he has a bloated stomach. I suspect it might be related to gallbladder/liver issues, and asked him to request an ultrasound from his doctor. But she gave him an abdominal CT scan. I don't like the idea of him being exposed to radiation at his age and given that he once had cancer (of the prostate). Should he go back and ask for another test without radiation (e.g. MRI, etc.) or is this really the optimal test to examine a bloated stomach?