the only thing I can think of that remotely resembles, right off the bat, is ASD (Atrial Septal Defect) and that's a long shot. Some of the things you described such as the panting (Difficulty breathing, aka dyspnea) flu type illness (Frequent respiratory infections in children.) has he had his heart checked? It's possible he may have had an transient ischemic attack due to ASD that has caused the issues with his tongue, gagging and inability to eat or drink. You did mention an appointment with a neurologist though, any MRI taken? CAT Scan? ASD can also be the cause of poor growth if that is an issue.
Another thing to check into would be a chiari malformation at the base of the skull. It can cause some of the symptoms you have described such as the difficulty eating, tongue control and gagging.
also, this:
bothers me. why has the doctor not admitted him for observation? I would be asking this question if he is dehydrating that much, that fast. That would have me placing a PICC Line to make sure he didn't dehydrate and make sure he is nourished properly.
just throwing it out there. Had this kid had came to my office, I would have sent him for a work up with a cardiologist and out for an MRI and Cat Scan looking specifically for transient ischemic attack or chiari malformation.
hope this may help, I hope it is resolved soon, my heart goes out to him.