mucus misery

I posted the following on another forum and so far haven't had any replies, so i'll give this forum a go..............

Pop’s Bowels

My-88-yr old father, who I am caring for, has had a bowel complaint for 10 yrs or more. He has seen many doctors and they all give the catch-all diagnosis of IBS. Over the last 4-5 yrs he has had a mucus leakage that seems to be getting worse. [related to the IBS?].
The mucus leakage is his most troublesome problem as the meds can control the diarrhea somewhat. It used to be that a folded tissue placed in his underwear did the trick, but leakage has been getting worse lately and he may have to go into adult diapers.
The leakage mostly occurs after sitting or lying down for some time, then standing, mostly after dinner, seldom in the morning.
He has had the standard stool tests many times over the years for parasites and bacteria and blood and always tested negative.

His medical history:
In his 30’s he had part of his small intestines removed during an emergency appendectomy that turned up polyps.
Five years ago, he was diagnosed with prostrate cancer and was treated by removal of testes. He has survived this and has consistent low PSA readings.
Has had 3 colostomies and some polyps were found and removed 3 yrs ago.
He fell and broke his right hip and shoulder two years ago and subsequent surgery disclosed moderate osteoporosis.

Meds he is taking:
Fosamax [for osteoporosis] for 2 yrs
Codene [for IBS and sleep aid]
Immodium and lotomil occasionally for loose bowels, helps with diarrhea, but no relief from mucus leakage

Tried antiflatulent [semethicone] and charcoal for short period and no relief

Vitamins and suppliments:.
mornings
Calcium w/vitD and magnesium
Multivitimin
500 mg C
400 IU E

Pm
1000 mg B-12
100 mg B-6
500 mg C

He gets regular exercise, often walking up to 2 kilometers a day.

His diet consists of [lots of] fruit and grains [oats&bran]
with flax seed and soy milk for breakfast.
Lunch of mostly brown rice and meat/vegetables and occasional pasta.
Same for dinner. Not low fat, but normal fat content.
Night snacks of banana and orange.
Drinks 1 cup of coffee and 2 cups of tea during the day.

Questions I have are….
Could it be his many supplements and vitamins causing it?
Could it be diet?
Could it be stress?
Could it just be an 88 yr old system failing and resign himself to adult diapers?

Any suggestions will be appreciated……
Thanks in advance