That sounds so frustrating.

To me it sounds like he might not understand how to clean it up on his own. Maybe you need to go through the processes of cleaning it iwth him so he's not wiping it on walls or carpet. It will probably take a couple of weeks before it becomes automatic for him. Also strategically place some nappy wipes, hand sanitiser and things like that around the house for him to grab if and when it happens. Maybe also make up a picture chart of the process and place one in the toilet that he can refer to if necessary.