It sounds like gastric stasis. Unfortunately, that is a symptom of another problem, not a primary cause. See
PetMD Mobile - Dog Health Article
" The signs and symptoms of stasis are usually the result of an underlying problem that causes the stomach to stop working. Such problems may include:*Stomach ulcersCancer of the stomachMedicationsStress, pain, or traumaInfection of the stomach or intestines (gastritis; enteritis)Obstructions or blockages in the stomach or intestineSurgery affecting the intestine or stomachMetabolic disorders of the body (anemia,*hypothyroidism, acidosis)Distention and flipping of stomach (gastric dilatation-volvulus syndrome or*GDV)Enlarged esophagus (megaesophagus)"
As a people nurse, I know an elevated bun with normal creatinine can be a sign of gastrointestinal bleeding. His lethargy could be due to anemia from blood loss. It sounds like you are doing everything right and your vet seems to be quite thorough. All I can add is maybe add a medication that promotes gastric motility such as reglan? Good luck and keep us informed!