You say he had a colectomy and no colostomy bag, but that doesn't really address what his new plumbing is like. He could have an internal pouch, like a j-pouch, in which case he might have pouchitis, which needs to be addressed by a doctor. If he has a straight connection, he might not have a lot of great options, but hopefully the surgeon created some type of reservoir for him.
If he's working with a doctor or surgeon, he can ask for prescriptions for Lomotil or Imodium or tincture of opium. He needs to up his fiber intake to at least 30 grams (25 for women) of fiber through food (beans, apples, chia seeds, avocadoes), fiber bars (look for bars like NoCow with at least 15-22 grams of fiber per bar) or supplements (psyllium powder). Probiotics: the only one that has gone through some clinical trials is Visbiome (formerly known as VSL #3), which is a refrigerated probiotic powder that has shown some promis in helping those with chronic pouchitis or ulcerative colitis. It's not cheap and isn't often covered by insurance, but if he's serious about trying a probiotic, I would recommend this one. Again, to get the clinical strength version, he will need a prescription and some patience to find a pharmacy that will order it.
If he's using Metamucil or other psyllium fiber supplements, avoid artificial sweeteners; Many of the sweeteners aggrevate diarrhea. And he needs to be reading the labels...It takes a surprisingly high amount of psyllium powder to make a difference.
There are people who never get a good quality of life after surgery and choose an ostomy. Quality of life is important, and it sounds like the surgeon hasn't done their job well if your BIL is still struggling four years later. An ostomy might be the right path to consider to help him gain freedom from the bathroom.
2009, age 37 with a 2yo: CC, IIIB (T4N1M0). IBD history
2010-11: FOLFOX, 12 rounds. 5 surgeries (3-step Ileal j-pouch, infection, VATS)
Mom, you can go the hard way if you want, but the easy way is much easier. ~my 3yo