I had a darling cat who peed on lots of things. He had urinary tract infections several times, including pretty severe ones, and also just peed around the house for no apparent medical reason. In the 10 years I had that little guy I learned a lot about cats peeing outside the litter box, so I’m just overjoyed to be able to share it now.
You definitely want to get a UTI ruled out first, but peeing around the house (including in the shower, I’d say) can certainly have other causes. As other posters have mentioned, it could be that his litter box is dirty. He could be feeling ignored, for example if there has been a recent change in his lifestyle like a new cat or a move to a new house. He could be marking his territory, unusual with a neutered cat but not unheard of. If you have other cats they might need more litter boxes because cats aren’t always good at sharing.
One of the worst problems is that once a cat decides it’s nice to pee outside the litter box, he may keep doing it just because. Even if you get the underlying cause treated, the habit may continue. The cat might also be peeing there repeatedly because it still smells like urine. The area can be stink-free to us and still smell like piss to a cat, so make sure you use the right kinds of cleaner … no ammonia for example.
p.s. By the way, if you want to know why I no longer have the aforementioned cat, it’s because he peed on my bed one too many times.