We have cats (plural), and cats do get stressed out.
First, you want to get him checked out at the vet's to make sure he is healthy. Cats can be ill without looking it, and this sort of marking can be a sign of a physical illness.
But let's assume he's nice and healthy and temporarily grouchy. There is a product you can buy at a pet store called Nature's Miracle. I buy it by the gallon. It's a little pricey (I look for sales), but absolutely worth every penny. I use it wherever there's a mark or a smell. Often I use a spray bottle, but sometimes I just pour the stuff on. If the offending place is on carpet, you have to let it soak through the carpet and the pad. You'll know you got the marked places completely if, after the product has dried completely, the smell is gone. I have also used it on furniture, and sometimes even in the wash when something has been very mistreated.
You might try using it to wash down every wall (the lower parts), the baseboards, and the floor - and anywhere else you can think of - in that bathroom. That room may need to be off limits to the kitty for a while as well.
Your stressed-out kitty may be smelling things that you don't and marking those places, or he may just be doing it to say, "This is MY turf and not any old visiting poodle's!" On the other hand, one of my kitties has taken to marking the bathroom rug - the one by the tub - and it's not because he doesn't have a clean litter box (oh, and make sure your litter boxes are very clean). He's just taken a fancy to doing it. I have to hang the rug on the side of the tub rather than leaving it on the floor when it's not being used.
Another thing I have done is to spend extra money for a professional carpet cleaner to put the professional-strength stuff on the carpets before cleaning them. That really taxed the budget for that month, but it was worth it in a particular room that the cats heavily abused. I made it off limits to them for several weeks after that, and I'm slowly letting them in again with supervision (it's a guest bedroom).
It can take a while to figure out what's in a kitty's mind. But pretend he's one of your kids and keep at it until you solve the quandry!