It's pretty common for a dog to become inclined to using the bathroom in a particular place. Even if they know to pee outside, it doesn't necessarily mean that they understand that they must poop outside as well. Unfortunately, your dog now associates the shower with being an acceptable place to poop.
It is essential that you restrict access to the shower completely. Your going to need to re-teach your pooch that this is not the right place for him to go.
Now that your dog has no access to his familiar bathroom place, he will be looking for a new place to go. Start taking him out on a routine "poop" schedule besides his regular pee times. Usually 20-30 minutes after a dog eats gives them enough time to digest their food and prepare for defecating. But make sure you do this after every meal time.
Until he his properly housetained, it is also very helpful if he's not left unattended. Dogs can't make mistakes if we don't let them. If you do catch him in the act, make sure to discipline him. You should also try using the crate method for times that you can't be with him.
Hope this helped!
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