Has the doctor ever mentioned adhesions? I would get a appointment with the surgeon or affiliate and have this checked out. Has he had a Colonoscopy recently? How is his appetite? Has he had weight loss? Does he have regular Bowel Movements? Any blood in the Bowel movements? Any vomiting? Please have this checked. Pain like this is not normal.

@kets
I am pretty sure his pain is related to gas in the intestines and the peristalisis that is pushing the stool through his intestines with scar tissue formed at the anastomosis. My surgeon did a colonoscopy and saw I had a stricture at anastomosis site and he blew up a balloon on the scope gently and stretched it out a little so I could more easily pass the stool. Microscopic air got out of colon and went to my right shoulder inside but it resolved on its own and praise Jesus my surgeon was conservative and didnt take me to surgery. This is a normal occurrence if only a little air.
After that it has been easier to eat soft foods ,no girsley foods, no steak. I actually felt normal for 1st time in 5 months. I stopped loosing wt. Could choose foods I like. My surgeon may have to stretch it again a little but at least I have hope. Ask your surgeon about this and let only a surgeon who has done this many times do it. Also diverticulitis must be controlled. I also drink daily prune juice 4 ounces and 1 teaspoon olive oil.
Prayers for both of you.