Might be time to listen to the sad opinion your vets gave you- to "let him go" as that's not a sustainable much less a good life for a snake- snakes are very stoic so he's probably feeling worse than you think, & his "activity" probably reflects his internal discomfort, & quite possibly pain. I'm so sorry to say that, & I'm not seeing him in person, just going by what you've said, but honestly, I'd consider euthanasia at this point. Unfortunately we cannot help them all, & we need to consider their suffering too, especially since it sounds like you've done pretty much all you could for him.
Remember that in nature, not all survive anyway- & if it is some internal malformation that's causing this, you'd not want to pass it on, certainly.
BTW, the only time I've tried digestive enzymes for a snake was a VERY different situation altogether- and they helped very little anyway. It was actually for a rattlesnake that couldn't digest after his former owner had his venom glands removed surgically to "make him a safe pet"- not considering the fact that venom appears to be the main source of their digestive enzymes. Some can digest without using their own venom (their stomach normally produces some digestive enzymes too), but this one clearly could not. Ultimately the added enzymes did very little for him- what actually worked best was to use a "venom donor"- I had one of my other rattlesnakes (fully venomous) bite his rodents for him, & happily that did the trick. The other rattlesnake thought I was a real jerk for stealing his food after he'd nailed it though, and oh the dirty looks I got!

But he got plenty more to "keep"- & he clearly had no shortage of venom anyway.