You've probably already done this, but it's worth sending off a few samples to check she doesn't have a urinary trat infection. Another occasional cause is threadworms, but at this age she'd probably be able to tell you if he was getting itchy at night-times.
Otherwise it probably is just on the normal spectrum, but at a time when things are stable and relaxed at home it might be worth trying the star chart approach.
Hope you're well organised with soaky-up sort of things to limit the washing..