hide all the food and slowly give him only water then move on up to food but only give him little servings so his digestive system gets used to that amount of food and then he won't eat so much.
So sorry to hear about your struggles.
It is likely that the pyloroplasty contributes to both the dumping syndrome and the gastritis. The dumping (there are two types, and you do not describe which, but "early dumping" would be likely in your case: cramps, jitters, bloat, diarrhea. "Late dumping" is a severe reactive hypoglycemia) happens when a smaller stomach (from the Nissen) encounters and incompetent pylorus. Gastric contents leave too fast, producing the symptoms (the small bowel reacts badly to too-high volumes of too-high concentration).
The gastritis may also stem from an incompetent pylorus--bile and strongly alkaline fluid from the pancreas and duodenum flow back into the stomach which has evolved to tolerate strong acid, but has trouble with strong base. This is very hard to treat, but some have reported good results (and I have seen this too) with Carafate. There are also several medical strategies that could be tried, some just involving changes in how and what he eats and what medications you use.
Reversal of pyloropplasty has been reported as successful, but is not at all common (fewer pediatric surgeons even do pyloroplasties any more for the reasons you have experienced). In an extreme case there are operations that could be done as well, but may be no better than what you have.
Ask your surgeon about reversal. And consider a second opinion.
T.