Can’t log my sons phone back in ‘Find My’

What most likely happened is that he opted out from sharing his location with you.

The only way to log back in is to have the Apple ID and password. I read that you have the Apple ID but do you know his password? He may have changed it to something else. If that's the case, there's no way to log him back in. He doesn't need to be logged in by the way to still track him. So he may have also unshared his location with you before he logged out.

But yea you'll need the Apple ID and Password.

Nothing is random; if it was logged off someone logged it off. Do you have his phone? Can you log in to his Apple ID on it?

You can take the phone away, or cancel service on it.