My son “wiggles his bum” not sure how to approach it

It's definitely normal to try all kinds of things with their body. You could set a boundary like mine wants to touch himself I say only in private

Totally normal. We frame it as private time and explain that we don't touch our privates around other people but you can go to your room and have some private time. I usually set a time limit and if he is doing it a lot we talk about not making ourselves sore

Oh my son would do that here and there but I’d just turn a blind eye tbh and he wouldn’t continue- It’s definitely developmental and nothing to worry about personally, With my son I’d put boundaries in place if it could come across inappropriate in front of others or towards others. Example redirecting his hand. Or if he’s try be funny regarding his privates waving it around saying it’s a helicopter (I had to stop myself from laughing) id say thats your privates you keep that to yourself and ignore after that.
If it does make u uncomfortable what id do is just redirect… “hey let’s go and stomp stomp stomp our feet” thats what id do anyway just distract cause 3 i dont think theres a way of making him understand boundaries there because he isnt actually doing anything wrong is that makes sense?