NOW Foods Supplements, Psyllium Husk Caps 500 mg, Non-GMO Project Verified, Natural Soluble Fiber, Intestinal Health*, 500 Veg Capsules

I use the NOW brand

If he's able to take capsules, this is a good fiber supplement. Drinking plenty of water also helps. My mom had issues passing stools all her life and would drink prune juice and take both fiber supplaments and stool softener to help.