20m old plays with his nipples (suggestions for how to stop)

Hi guys!

So right before Christmas my baby started to play with his nipples, mostly when he was relaxed watching a show or snuggling. But now it's ALL. THE. TIME. non stop, all day long. Even while he's running and playing!

I don't want to yell at him, I've recently gently grab his arm and pull it out from under his shirt than distract with something.

I'm thinking about getting onsies for him again to see if that stops him from getting to them.

I'm just curious if anyone has suggestions

Also...if you're going to say leave him alone, you'll damage him for correcting this behavior etc, save it.

Thank you in advance!