I don't mean to freak you out more then you already are..
But since you are going to the Vet tomorrow, and that is the time to bring up and discuss all the possibilities and to order any tests, I'm going to link you to some other possibilities, the "dark" ones when it comes to reoccurring URI in snakes.
Of course it DOES NOT mean he has any of those. Its early days yet, he could simply been having a bad day. But again, since you are going to the Vet, might as well question about the other possibilities. Especially since you have other beloved reptiles, and some diseases are air-born.
In the mean time I would make quarantine as strict as humanely possible. I know you are already taking all kinds of precautions. But I would want him and his cage in a completely different part of the house. True, it may not make a difference at this point, but what can it hurt.
Hopefully you will get more (and better) answers tomorrow.
https://skiploder.wordpress.com/2014...ris-in-snakes/
I'm crossing fingers and toes that he just needs a bit more time and will be right as rain before to long !!