Start with getting your father to a doctor for a check up. We on this forum are not physicians. Has your father been diagnosed with anything gastrointestinal in nature? If not then he needs a work up. Good luck!
Our doctor was able to make suggestions on foods to avoid, foods to add and meds that might help. But these were suggestions after a medical work up. My husband is doing much better now.
Not medical advice...but I had a bug or something a few weeks back and that led to my seeing multiple articles/ reports of the benefits of macaroons...the coconut type. Perhaps I was already on the mend, or maybe it was the benefit of the coconut...but I picked up a pack at Aldi's and was eating one a day...and things improved...Sounds like you have more to explore though with this situation...
If not then he needs a work up.
Good luck!