What should I say and what can we do/what should I tell him to do so that it doesn't happen again?

Pretty much... "hey, please don't do that again" ...?

There's this awesome thing that adults have in a steady relationship called "implied consent" - it basically means that it's implied that unless explicitly stated otherwise, that either partner has the consent of the other to try to initiate physical contact/sex/etc - on the understanding that they cease if the other person says no. It's how the real world functions.

I know the current zeitgeist is that everyone is meant to stop and ask for fully informed consent at all stages, but that is not how real people who are in a steady & properly communicated & negotiated relationship actually live, nor should it be, it's fucking exhausting to double-think everything - and why bother when you can just have that conversation once?

So open your mouth and just freaking TALK TO HIM.