Digestive problems two years after successful colon cancer surgery....

Hi! I am new to the group and am happy to be here. I am wondering if anyone has had a similar experience to our situation: My "young" 77 year old husband had colon cancer in 2013, with successful surgery (he no longer has a colostomy bag which he had while he was in chemo & radiation). He has always had a somewhat sensitive digestive system but it did not interfere with his very active professional life.
He has since been diagnosed with FTD- a degenerative brain condition which brought along some Parkinsonian type symptoms which has limited his walking so he sits much of the day. For the past couple of years he has increasingly had daily problems with digestion- most often constipation, the next day diarrhea. As soon as I get it balanced out to normal - solid well formed bowel movements, within a day or so it will be the opposite. I try to be very careful with diet. We eat healthy, mostly organic foods. I give him a bland brat type diet when he's "loose", and more roughage when he's constipated.
but it seems to be a never ending cycle....