How to stop someone from urinating on the bathroom wall?

I live with my unmarried 30 year old brother. He urinates on the bathroom walls and uses the nearby hand flush to wash it off. I have to shower next to this wall everyday.

Bathrooms here have a single drain hole, a shower and a toilet with flush system. There is no bath tub.

As disgusting as it is, my father has a bad habit of doing the same thing. My mother urinates on the bathroom floor and then tosses water. Though, they don't live with us. I have a feeling my brother has picked this up from them and continues to do so because despite my repeated complaints, they have never talked or rebuked my brother about this. Their quiet on this encourages him to continue this behaviour. My parents change the subject/get embarrassed/console me with some stock responses upon broaching this topic.

I don't have the option of moving out just yet.

How do I stop or discourage him from urinating on the walls?