Hi,
It sounds as though he is experiencing Gastric Dumping, where:
"the upper end of the small intestine, the jejunum, expands too quickly due to the presence of hyperosmolar[jargon] food from the stomach. "Early" dumping begins concurrently or immediately succeeding a meal. Symptoms of early dumping include nausea, vomiting, bloating, cramping, diarrhea, dizziness and fatigue. "Late" dumping happens 1 to 3 hours after eating. Symptoms of late dumping include weakness, sweating, and dizziness. Many people have both types. The syndrome is most often associated with gastric surgery.
It is speculated that "early" dumping is associated with difficulty digesting fats while "late" dumping is associated with carbohydrates.[citation needed]
Rapid loading of the small intestine with hypertonic stomach contents can lead to rapid entry of water into the intestinal lumen. Osmotic diarrhea, distension of the small bowel (leading to crampy abdominal pain), and hypovolemia can result.
In addition, people with this syndrome often suffer from low blood sugar, or hypoglycemia, because the rapid "dumping" of food triggers the pancreas to release excessive amounts of insulin into the bloodstream. This type of hypoglycemia is referred to as "alimentary hypoglycemia"."
Regards
Simon
Source: http://en.wikipedia.org/wiki/Gastric_dumping_syndrome