psyllium husks. (the same fiber that's in metamucil - minus the flavoring) i suppose you can use metamucil too. either way it adds bulk to the stool, which should increase bowel motility. make sure he drinks lots of wqater, to help the psyllium bulk up. prune juice works too, but at least from my experience more like a laxative. i've actually just had a recent experience with one of my cats, of all things, having a constipation problem. the vet gave him stool softeners, and sure it made his bms softer, but did nothign for volume, so he stayed constipated. i add psyllium husks to his canned food, and now he's got smooth moves every day. and his coat has gotten much better. in any case i would suggest psyllium husk fiber, before going the prune juice route. or perhaps psyllium fiber with water at night - and then prune juice in the morning, so that it really cleans him out that first time.