I think the only way you would know what if you took him to the vet for a "snake autopsy" it starts with an N, but I can never remember what it's called. I believe you have to put the snake in the freezer right away or the vet can do anything. Someone else would have to chime in.

If the rodent wasn't enormous from the mouse then he couldn't have been killed by it. They can eat something a bit larger than themselves. If the mouse was f/t it could have been a bad mouse-left out at the store or thawed while being shipped. If your snake didn't eat for a long time the shock of eating can poise a potential hazard if they are really weak. It's just like any animal, and I believe humans, if they are starved for awhile and you let them eat a big meal their bodies may not be able to handle it and shut down from the shock. I don't know what your situation is, but that's what I know.