I agree with Shanti...always focus on the positive. The clean up should just be matter of fact, no big deal, the same as you'd expect him to clean up a non-deliberate mess he's made. Put the toilet rolls out of reach! Maybe also let him know that if he keeps putting toilet rolls in the toilet, you'll have to take it away and he'll only get two sheets next time he wants to go to the toilet...? (So, just leave two sheets out with no roll). Can you see it coming? ie is he upset about something else, tired, hungry, looking for attention etc? If so, can you head it off with distraction? Lots of positive time spent together helps here too.