Is this a poop or regurg?

[Bogertophis]

Right now though, I'd make SURE all his temps. are where they should be.

Wait at least 3 weeks before you offer him any food again, & only give a smaller meal than his previous one. You might also try getting it from a different source, as sometimes there are issues with the quality of the rodents themselves, but from the looks of that regurge, there was barely any digestion going on...so I don't think it's likely to be a rodent issue...UNLESS it was too big for him, & you haven't shared that information either- his size (weight & approx. length) compared to size (weight) of prey.

Let him rest in the meantime- minimize handling- don't schedule a vet visit until you find out what happens after his next meal. IF he digests & keeps it down, fine...go easy on his meals for a while, no vet needed unless he regurges again, or has other symptoms. You REALLY need to avoid repeated regurgitation in a snake, as it can actually kill them. They can aspirate, or become dehydrated.

And speaking of meal size vs. size of snake: