What breeder did you get him from? Have you contacted them to see if any of their other snakes are showing symptoms? Did you get the snake at a reptile expo?

Where did the rodents come from that you fed him? How old? Live or frozen?

If he had dislocated a vertebrae then he would be paralyzed from that area down. He wouldn't be able to breathe.

Boas from US collections are able to be shipped over to Europe, so it's possible(just covering bases) that an IBD carrier infected him if you got the snake at an expo.

He may have been exposed to another type of virus or more resistant strain of bacteria. It could also be poisoning, maybe from what he ate.

Whatever he has, if he cannot hydrate himself(drink), then he's not going to do well. Your vet may want to do a blood culture just in case. Keeping him hydrated is very important right now.