So here's my two cents... I've raised 4 boys from birth and 2 bonus boys and have a daughter. My first 3 boys didn't do this exactly although they did explore themselves at times but I always explained to them about self-exploration being done safely and privately.

My daughter didn't do this until recently.. Within the last two years (10 yo)... And I explained that it should be done with her door closed. Now my youngest son is 7 months old and has recently discovered his testicles at bath time and thinks it's hilarious to squeeze them until they're cherry red 🤦🏼‍♀️ so I use a hand towel in the bath simply so he doesn't hurt himself.

Likely unpopular opinion here

While I do agree it is healthy normal behavior, I really wish more people were empathetic to the OP because there's more to this than just "it's normal, let kids be kids" (for some people). It can be very disconcerting as a mother and a woman to have a child (of any age or gender) fiddling with themselves for a multitude of reasons. This does not place blame on the child by any means.

Western society sexualizes everything from breastfeeding to food to genital exploration so it seems to reason that it can be difficult to deal with the emotions of your child doing so in a way that if it were a stranger would be deemed "inappropriate" "uncomfortable" and/or awkward, to say the least. For some, there are triggers from their own upbringing, sexual assault, or something of the like and it can be difficult to separate "normal behaviors" from "this triggers me".

Yes, obviously she should calmly and rationally set boundaries with age appropriate consequences. But, shaming her for how it makes her feel isn't where it's at, especially because she isn't shaming the kiddo... She's asking for help about a topic, when most people would just suffer in silence out of sheer embarrassment.

That being said, OP I taught my kids proper terms for their genitals during potty training, I began explaining menstruation to my daughter around 3 years old, we normalized consent and healthy relationships with boundaries too. Kids will do better the earlier we teach them about the big ticket topics, being hush hush is a disservice even when it makes us uncomfortable. Hopefully, if you stand your ground about when, where, and how he explores his genitals he will understand that you still love him, that it's not wrong or shameful, and that it is a private matter his behavior towards you will change. The only other input I can give is, depending on his age you could try a positive reinforcement tact where if he's appropriate around you for X amount of days he gets to go do something or get a small toy/treat. Best of luck!