See a specialist, either Internal Medicine or Neurology. Your dog has not responded to the treatment from the regular vet.
He will probably need diagnostic testing to be accurately diagnosed.
Have not watched your video. There are no veterinary healthcare professionals here and even if there were, they have not examined your dog so they would not be able to give you specific advice.
I would make an appointment with a specialist, asap.
Several gulping threads are here/not closed. Just use the search engine, example
/forums/topic/frantic-lapping-gulping-licking-whining/
PS: Just watched the video. Impossible to speculate on what it could be. There are a multitude of disorders that can cause these symptoms. I would not give over the counter meds, supplements or any other remedies unless prescribed by a veterinarian that has examined the dog.
