Dad suffers from alzheimer's complaining of pain in stomach & its become swollen markers for infection?

Hard to say. Alzheimer's doesn't cause belly pain or swelling. Serious conditions like liver problems and less serious problems like constipation can cause these problems. People with alzheimer's often cannot give an accurate history but they can tell you how they at this current moment. Best to call or visit his primary care doctor.
Get him examined. He might have a serious problem and should be seen by a physician.