Corn syrup itself is not problematic, but it is possible that it is "high fructose corn syrup (HFCS)", which can cause diarrhea in fructose malabsorption.
Avoid: any fruits, any fruit juices, honey, corn syrup and sorbotol (sorbitol is a sugar alcohol, on labels it is referred as polyols). He can have table sugar but in limited amount, since it contains fructose.