My son has an iPhone 6s and I'm trying to find a way to monitor his texts without modifying anything on his phone because he's fairly tech savvy so he'd be able to remove it. I was thinking about a way to spy the texts when they go over wifi. Is there a possible way to do that?

I guess iKeyMonitor might help.
https://ikeymonitor.com/

I don't think so except you can snatch his phone and see his messages !!