The next time he went, I put a little bit of dishwashing liquid in his potty. (pottytrainingconcepts.com)
If more thorough cleansing is required, use a solution of mild dishwashing liquid mixed with warm water. (houzz.com)
Another trick I just learned from my contractor after having my sink replaced is to occasionally run some Dawn dishwashing liquid with very not water down the drain. (consciouscat.net)