re: Son getting a ton of spam texts on his phone-how to slow this down

Go to Settings > Messages > turn on "Filter Unknown Senders" (it's near the bottom)

It won't do exactly what you're looking for, but when he receives one of those texts, he will not get any sort of notification about it, and it will be sorted in a completely separate area of the messages app. Will look like this:



Think of it like a "Spam" folder in your email. You'll need to go in there occasionally and delete them all, but at least he won't be bothered all day.