Some cats don't like sharing litter trays. By having two or more as others have suggested, cleaning up all wee's and ensuring no scent remains [very difficult since you can't smell it] may just help. Is there a particular place he likes to wee in? That might be the spot to put a litter tray. I would also suggest you have various trays dotted around the house to encourage appropriate toileting.

It could also be a case of habit now and once developed its hard to break. Have you thought about a weeks break in a cattery? That would be totally new sourroundings in which he had no preconceived ideas about toileting and being totally on his own in the pen might just stop this behaviour.

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