At first I thought it was floor and walls everywhere in your house, but as I read on, I realized that you meant of the bathroom.....right?

Definitely keep the door open when they both go. Maybe have your son only use a different, family bathroom....like if you have one up where the bedrooms are. This will eliminate him from any accusations or questioning. It will ALL be on the dcb...no excuses.

I would also insist that he sits on the toilet...no standing allowed.

And for now, I would absolutely accompany him. He must ask you and go with you. After a certain time, he can earn going alone again, but of course with the door open.

Strange. Good luck! OH! And of course he should be responsible for the clean-up. Although, I'm sure you'll redo it after, but he still needs to understand that he made the mess, he cleans it!