Maybe an idea to provide an additional litter tray and/or change to a different type of litter. You could also try using a little Bach's Rescue Remedy - put a couple of drops in his water to help relax him.
The other thing is honeysuckle spray - this is an alternative to catnip but it does have a different effect on different cats. I sprayed some on a blanket of a cat who hated travelling in the car (to vets) and it totally
chilled him out but on a couple of my other cats it does have the same effect as catnip - gives them a high and makes them excited. It doesn't smell much to us at all but the cats can smell it.
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Good luck; I hope you can sort this out for JJ.