I am sorry that your son has suffered so much from his type 2 diabetes. His condition certainly sounds complicated and should be managed by specialists. The first and most important therapy for diabetic gastroparesis is to manage blood sugar levels, which it appears he has done well with. The second treatment we use for gastroparesis is medications that can accelerate stomach emptying. These include the antibiotic erythromycin and sometimes the anti-nausea medicine metoclopromide. The third treatment that can be tried in these patients is something called a gastric pacemaker. This is a device that tries to replace the natural enteric nervous system by opening the outlet to the stomach at the correct time to release food into the small intestine. A small bowel and stomach transplant is an extremely invasive procedure that is reserved for patients who's digestive system is so inadequate that they require intravenous nutrition to live. I have never heard of a case of diabetic gastroparesis that was treated with transplant. I imagine that the side effects of the procedure would be unacceptable unless his condition made nutrition by mouth impossible.
I suggest that you have your son see a gastroenterologist
for his gastroparesis. He or she can review what advanced therapies would be helpful for your son's condition. Good luck.