[kalina82]

I wouldn't take him to the vet just vet. its most likely the cat poop that is causing the problems. Since you've blocked his access to the box i'd wait a few days. If he starts having diarrhea or something else starts up, then i would take him in.

Is your kitty on a monthly dewormer? if not, then i would take a sample of bernies poop to the vet to make sure he hasn't picked up anything from eating the cat poop. The only reason i say this is because your cat is a mouser.