What Causes Diarrhea Or Cold Sweats Every Time After Eating?

My dad is 74 has no appetite, when he does eat he either has diahrea or cold sweats. For the past 7 days he did not eat but drank 3 Dr Peppers a day for 5 days yesterday he ate some ice cream got diahrea then today had a fried chicken thigh and half of a baked paotato, and got the cold sweats. What is wrong with him? His previous diagnosis 6 months ago was prostate cancer and he has conjestive heart failure