Does he actively retch (with stomach contractions) before each vomiting episode, or does he just open his mouth and food comes out? It doesn't matter if he retches after the "vomit" but it does matter if he retches before.
X-rays don't necessarily show everything your dog eats. I've seen lots of dogs with problems that had something floating around in their stomachs for months before the problem was resolved. The barium is not only a good test for the esophagus problem, but it can also pick up things the original x-rays did not show.
Dogs with mega-esophagus only lose weight if they are not keeping enough food down and can't meet their caloric needs. A mild case will not lose weight, so healthy weight does not rule out megaesophagus.
I think your vet is doing a good job, I don't see any reason to go elsewhere unless you are otherwise unhappy.
Allergies does not usually cause these symptoms, so I really think you should continue with the GI work-up.