Coffee is a diuretic, so be sure he doesn't drink more than one cup a day. Sodas, tea and alcohol are also dehydrating. Water and juice really help to move things along.
Is he on pain medication? Those also cause constipation.
I recommend soluble fiber supplement in powder form. I mix it with juice and drink it down. It really helps me a lot. I also find that spinach, cabbage and bok choy empty me out pretty well too.
My doctor recommended 64 ounces daily of water and juice. If I drink tea or soda, that should not be included in the 64 ounces. Fiber and hydration are really the keys. I avoid rice, red meat and cheese since they tend to stop me up.
Hope some of this helps!