I know it's very hard to give up when you keep thinking you see signs of improvement. And btw, I'm assuming you've triple-checked that he has adequate warmth to digest? Have you tried bumping just a few degrees higher than most BPs require? (but staying within safe parameters, of course) And you've been only feeding small prey, right?
From what you've just described above ("...eats great...then 2 feeds later throws it back up...) it sounds to me like there's some sort of partial obstruction* that sometimes allows food to pass, but keeps getting caught & again blocking the passage at other times. Like a small tumor or other malformation -like a flap- that gets caught & obstructs? But again, to put a snake thru major GI tract surgery to first find & then repair it is just not something I'd do- because it's so much risk & long time suffering for such a slight chance of success- & again, that's assuming you could even find someone with the skills, never mind what they'd charge. I personally wouldn't put a snake thru that, but ultimately it's between you & your vet, & I wish you the best no matter what you decide.
One other thought- what does his stool look like? I'm assuming he has passed some now & then? I've actually heard of snakes born or hatched without an opening to pass wastes from their body but it's rare & they wouldn't have lived this long. Snakes can get blocked up from abnormal tissues like tumors & strictures, or from urate stones or undigestible debris, & if he's unable to expel wastes from his body for whatever reason, that alone would make his meals come back up. When he has upchucked his meals, I assume they were actually from his stomach (ie. vomited & not just regurgitated without digestion)? ***How digested were they?*** (How soon after eating did they come back up? I'm still wondering if digestive enzymes could help?)
I'm also assuming your vet has plenty of experience with snakes? Please do give us an update- so sorry you're dealing with such a tough problem.