He's fully potty trained for 3 years and only started doing this 4 months ago.
Water play or sensory play on YouTube just google might help
With my kids we talk about how we don’t touch in the toilet.. It takes a lot to keep (me) from freaking out! But the less I freak out and the more I talk calmly about how we don’t touch in the toilet & how we have to wash wash wash the less they do it 🍀
also my son have this problem, in addition he want to visit each bathroom on every mall we go.. very stressful