4yr old obsessed with his penis

Y'all I've been struggling with this for as long as I can remember. This child has been obsessed with his Weiner since he realized he had one. And I feel like he's gotten to the age where he should know not to touch himself when people are around but NOPE.

We were snuggling and watching a movie and he just starts playing with his weiner through his shorts. I've told him over and over again and he won't listen. I know it's not sexual but he needs to understand it's not appropriate.

I give him the option of "if you wanna play with your penis go to the bathroom, I don't want to see it those are your private parts" I've had this conversation with him a dozen times.

I'm about to have another baby, a girl and I really don't need him rubbing his penis in front of her 🤦🏻‍♀️ like does this ever stop? When does it stop?