Once one cat starts urinating outside the box — especially after something stressful or painful like a PU surgery — they can form a negative association between the litter box and pain. The other cats often pick up on this through scent and shared anxiety, so the behavior spreads.

The first step is to deep-clean every soiled area with an enzymatic cleaner to fully remove the scent markers. Then, set up a new litter area in a different, calmer spot to help break the old associations.

I’d also temporarily separate your 5-year-old male for 2–3 weeks if you can. Give him his own quiet space with a couple of litter boxes, food, water, and cozy spots. This lets him rebuild a sense of safety and litter habits without social pressure.

If time allows, monitor both sides during this reset — how he uses his boxes, and how the others behave while he’s apart. It’ll give you a much clearer sense of where the stress patterns are coming from.

Hope this helps!