Receiving copies of your child's text messages?

My husband and I are concerned our son is texting people he shouldn't be. Since he knows we monitor his cell phone on occasions, we feel he is deleting certain texts or all threads he doesn't want us to see, making it seem like nothing is suspicious. I heard you can get a copy of your child's text messages, but I can't seem to figure out how to set it up. I get a copy of his texts, but he can get a copy of mine.