Maybe - maybe not. Things like this can get wrapped around the intestines, and cause them to bunch. Monitor the litter box. Hopefully it will pass. If you see a turd clinging with rubber band stuck in the anus, cut it off - do NOT pull. Check each and every poo that comes out, by squishing it to look for the rubber band (yes, gross. Use a plastic bag to do it). Monitor his eating and drinking and behavior. If he stops eating, starts vomiting, or there is NO poo in the box, then call the vet as that means he may have a bowel obstruction. If the rubber band is not removed, it will cause the bowel where it is wrapped to become septic, and then gangrene can set in and that will be the end of your cat. In the future, do not leave anything like this hanging around - rubber bands, hair ties, string, ribbon. If he ate one once he will do it again.