Some other things to consider along with a diet modification and slippery elm bark would be digestive enzymes (like Prozyme) and/or probiotics (Natural Factor's makes a good one called Ultimate Multi). Could be his intestinal flora is outta whack from stuff he was eating while on the street. The digestive enzymes will help him process the large volume of food he's eating until his own system finds the right balance. Neither should be used indefinitely.
"To close your eyes will not ease another's pain." ~ Chinese Proverb
“We must not refuse to see with our eyes what they must endure with their bodies.” ~ Gretchen Wyler