Take him back to the doctor. Make them test his CA19-9 to make sure the cancer hasn't returned. And when they tell you, no try this, this and this first... You tell them YES, NOW. Demand it, you can't afford to waste time if it has returned.
That may not be what you want to hear; but (if) that's the cause of the symptoms; the sooner you catch it the greater your chance of stopping it/treating it.