I am sorry to hear your husband is struggling. I struggled a lot when I was going though radiation for rectal cancer. My nurses suggested a low residual diet - less to come out. I ate pasta, kids cereals, dairy products and cooked, very cooked, veggies. I did not eat any whole grains or raw fruits and veggies. I was also prescribed anusul which worked much better than over the counter. Other than that, I just knew we're there were bathrooms. :-(. I hope this is temporary for your husband. I have since had an ileostomy and had it reversed. My bowels still aren't quite right so I can understand his frustration and pain. Sending positive thoughts your way.

After my resection (before breast cancer) I had the same problem. Not exactly the runs, but my gastroenterologist called it chronic dumping, not to be too graphic. It sucks and I am on Disability for it. Going through breast cancer was sure hard dealing with this issue too. What helps me is Metamucil. But don't take much liquid with the capsules. It provides bulk, which is easier to hold. I am a bit better because of it. You have to experiment to figure out what helps him. I now take 2 capsules at night & 4 in the morning. Good luck to him.

https://www.whatnext.com/experiences/search?utf8=%E2%9C%93&search%5Bquery%5D=radiation+proctitus
Searching our experiences tab brings back the above list. You might read through some of those answers, maybe some can help.