The bad news is, he's not missing. He's intentionally doing it, and it's called spraying. That's exactly what my Ginza does, THANKFULLY he does it in the litterbox area only. Since yours' only doing it in the shower litterbox, hopefully it's easier for you to clean.
When I catch Ginza doing it, I calmly say "NO please." and push his hind end down gently to the box. You don't want to spook him in his bathroom which would lead to him doing it someplace else. Hopefully after 3 or 4 times, he'll get the message that that position is not a desired one. Ginza still does it, but only occasionally, and I'm building a special litterbox area with linoleum so it'll be easier for me to clean and I don't have to keep lining the area.
One suggestion I've heard about, but haven't tried, is coating the area with aluminum. Apparently they don't like the sound when they do their business.
Hopefully others will have more suggestions. If anything else comes to mind, I'll add it. Good luck.