I found quite a number of articles that say that dumping is found in 10% of fundoplications.
A good one was on Emedicine (webmd) if you just google fundoplication and gastric dumping you'll find it second one down. I would print the thing out and hand it to your GI doc. It's long and fairly technical, and he probably hasn't read it. There are some medical treatments!
"Clinically significant dumping syndrome occurs in approximately 10% of patients after any type of gastric surgery. Dumping syndrome has characteristic alimentary and systemic manifestations. It is the most common and often disabling postprandial syndrome observed after a variety of gastric surgical procedures, such as vagotomy, pyloroplasty, gastrojejunostomy, and laparoscopic Nissan fundoplication. Dumping syndrome can be separated into early and late forms, depending on the occurrence of symptoms in relation to the time elapsed after a meal. Both forms occur because of rapid delivery of large amounts of osmotically active solids and liquids into the duodenum. Dumping syndrome is the direct result of alterations in the storage function of the stomach and/or the pyloric emptying mechanism."