I would just tell him to stop and go wash his hand. If he asked why? I told him it’s not good to be touching, playing, with it because it can give you an UTI.
He would ask why? I would tell him because that’s where the pee comes from and there are germs on your hands, so if you play with it, touch it a lot, especially if you are not going pee, then germs on your hand can get in where the pee comes out and thus you can get an UTI.
UTI are often caused by e.coli. Yes, my kids wash their hand after bathroom use, before and after meals, etc but they are also kids and sometimes I don’t think they wash good enough. So what I’m telling my son is factual especially if he has traces of e.coli on his hand. So in general it’s better not to stimulate your genitals for “good” hygiene and health reasons. That’s all the my kids need to know at this point.
Do I catch my 9 year old still doing it? Yes occasionally, however it’s less and less and every time I spy him I remind him NOT to do it for the health reasons. I reminded him how an UTI can be really bad. His sister my 8 year-old got an UTI so bad, last year that, she ended up in the ER due to dehydration. My son knows and remembers this. So UTI are serious business and we don’t want to do something that could make us sick.
His sister ending up in the ER due to an UTI is what really nip it. We were making process before that and it was becoming less and less especially seeing I was making him wash his hands every time. However seeing his sister end up in the ER really put an end to it. He realized then that Mom was right UTI are bad things.
Of course as he gets older stimulating the genitals well not be as innocent, because there WILL be sexual pleasure attached to the action as well. When SEXUAL pleasure is attached to genital stimulation then, in my opinion, the action become masturbation and is a sin. Right now, it really is just innocent touch. A touch that should be avoid for hygiene reasons. We don’t pick our nose; We don’t play with our genitals.
Edit Add: boys can get UTI too. It’s not as common but it is possible. It’s believe that girls get them easier because the urethra in girls is shorter then in boys.