Boys and masturbation

Hello moms! I have an 18 month old and he’s touching him self often. He will put his hand inside his diaper. Doctor told me today it was normal and that boys are exploring and that it’s totally normal and common at his age. These are the little things moms don’t talk about and I want to know if your boys did this or do this?

And no he doesn’t have rashes or anything he’s very clean and doctor checked him out today.

My concern is he will do this when we’re out in public and I don’t want that to happen. How can I prevent it and what can I do.

So far I will tell him firmly “NO!” and he seems to listen but then he will do it again.

Help mommies!