Brief Answer:
Stool examination & lower GI endoscopy necessary

Detailed Answer:
Hi,
I am Dr. Prakash HM and I would like to answer your question.
Abnormal eating pattern has possibly led to the weight loss. If he suspects ulcer, he should have some amount of pain around the stomach area, before or after food and based on history, it is negative. Because of drugs like Acetaminophen, it could be a possibility if the drug was taken for a long time.

Stool examination is the most important thing to be done first to identify any source of infection or for presence of blood. Due to abnormal eating pattern, he could be suffering from severe mal-absorption leading to defective absorption. Mal-absorption or Irritable bowel disturbance cause can also be evaluated by stool examination.
Otherwise, other causes associated with Gastro-intestinal disturbances along with weight loss needs to be evaluated. Upper GI endoscopy to rule out any pathology in stomach and Lower GI endoscopy to rule out any pathology in the colon can be done.
The chance of an Ulcer in stomach or duodenum without abdominal pain is rare. It appears to more of Colonic pathology.
Other than medications described, is he on any other medication? There appears to be no specific medication for dementia.
You can discuss with your doctor regarding the diagnostic investigations of stool examination and endoscopy (Particular lower gastro-intestinal or colonoscopy). If diarrhea is too severe anti-motility medications can be taken. You can take a prescription from your doctor. Without ruling any infection, I strongly advise you to not take anti-motility drugs.
People with dementia have memory impairment, and history could be incorrect. So a caretaker in this period is necessary to evaluate, and give a proper history.

Hope I was able to address some of your concerns. Good history and stool examination if available, i can provide more clarity to the questions asked.
If you have any further questions, do ask again. Happy to help you.

Dr. Prakash HM

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