The intestinal system slows down as well as the liver and pancreatic function with advancing age.
Since the patient is bed-ridden and is 90 years old I would advise you the following for the gritty black specks and hard flakes:
Get the X-ray of abdomen to see for the fecolith formation and possible impaction
in large bowel.
If fecoliths are present then saline-soap enema can help for complete evacuation and may help regularize the bowel.
Secondly, get the stool tests routine, microscopy and occult blood.
Food adjustments is a necessity to have normal stools.
Addition of pancreatic enzymes an probiotics may help further.
If there is occult blood present in stool, there may be need of a colonoscopy
to rule out cancer.
Tests of blood: routine and for tumor markers.CT scan
of abdomen if possible and further management as per the cause found.
Hope I have answered your query. Let me know if I can assist you further.
Dr. T Chandrakant, General Surgeon