"Harry, the shared toilet situation isn't really working well. I've noticed it recently and been getting complaints from the other guests that the toilet situation isn't working that well.My cleaner comes regularly but not to wipe up stains and spills, and because of this may I remind you that guests are expected to tidy up after themselves as a curtesy to others. You can read this in ....."
Something to that extent.
If it doesn't improve much or enough that day or the next, message him through the Airbnb message thread to keep a record of this in case of later problems. Simply mention the situation again and that you'd like to repeat yesterday's agreement etc. etc.
Then think of how you want to handle the situation (extra cleaning paid by him?), and keep everything on the message board - reiterate conversations, so that if Airbnb needs to be involved at a later point you'll have proof the problem didn't arise out of the blue or was a figment of your imagination.
Hope this helps.