This sounds like a blockage. Has your vet had an ultrasound or xray taken?

I am not familiar with either of the facilities you have taken him to - but I do live in NJ.

Get him to either Red Bank of Oradell. They have the most current equipment - and are the two full service hospitals in the state.

Don't let them operate without exhausting all the non invasive testing options!

Good luck, please let us know how it turns out.

Did the vets do a barium series to see if food is actually passing through the digestive tract? Is there any diarrhea involved, or just the vomiting? Do you have him on a bland diet? small meals? What treatment protocol have you been offerred? Is he vomiting just water or also food?
Dogs exhibiting the symptoms you describe (no diarrhea but vomiting), and having negative xray and ultrasounds (meaning finding nothing), could mean they are suffering from a condition known as mega esophagus. What this means is that the rythmic contractions of the esophagus are weak and so food and liquid do not pass into the stomach properly and end up being vomited back up. This condition can be managed by feeding multiple small, wet meals daily, but there is no cure.
If you forgot to mention diarrhea as one of the symptoms, then I would be leaning more toward a severe case of pancreatitis. Treatment then would include meds and a very bland diet.
If it were my dog, I would try every kind of non-invasive testing before submitting my older boxer to a possibly unnecessary surgery. Xrays or the unltrasound should have shown if a tumor was present. DO you know if a veterinary radiologist reviewed the xrays? at our clinic, we have radiology rounds every two weeks, or order a stat consult with a local radiologist if a dog is seriously ill. WHile general vets can read basic exrays, the expertise of the radiollogist can pick up things a regular vet may miss in an exray.
If the only time your dog is vomiting is after drinking water, then it sounds as if they are drinking way too much water way too quickly and vomiting it up. In this case, I would limit access to water to only small amounts at a time so that the dog can not overdrink. Ice cubes can also be a way to get liquid into them slowly to quench their thirst without causing the vomiting. How did the vets determine the kidneys are functioning normally? Did they do a blood panel? Have they suggested the possibility of diabetes?
More info on exactly what your vets have done and told you will help. Anyway... here are som ideas off the top of my head. Ihope they help.

Mega can and often does occur in young dogs since it is often congenital. However a dog with a mild case of mega may show progressivley worsening problems as they age and the muscles generally lose strength.