The vet just called and said she decided to do the necropsy tonight rather than wait. It turns out the snake was impacted (she said she didn't find any evidence of substrate, so he didn't eat any aspen). There was apparently a lot of puss around the impaction and stuck to the intestinal walls, and there were signs of infection.
So, that would definitely explain why he regurgitated and then refused my second attempt to feed him, and since he must have been feeling so ill that could be why he wasn't drinking.
The impaction would have had to occur from the last time he was fed before coming to me, but since there was no substrate, the vet said it seems he just wasn't able to digest , or possibly there was an infection already hindering digestion.
To me, I don't think this was anyone's fault, and nothing either myself or Travis could have predicted happening. Definitely just a very unfortunate fluke.