Calcium supps can cause indigestion if taken with meals(bufffers acid production). Calcium is just plain hard to digest. Excess calcium can create a favorable alkaline environment for fungal candida and cause constipation. Careful with overdoing the antiacids especially at mealtimes. You don't want to buffer too much acid then that can lead to indigestion. If he does chlorophyll or greens powder=in between meals/empty stomach. Some can have issues with chloropyhll supps.
If he has a stomach sensitivity I would go with new chapter brand or megafood. These brands seemed to work better for people with sensitive stomach/gut. This is the feedback I got when I was vitamin manager at a health food store.
Some enzyme supps can cause GI irritation for people that are have sensitive guts.