Sounds like there are underlying trust issues. Why do you feel the need to go thru his phone? What's the nature of his relationship with his son? Sometimes men do stuff thinking they can make a better impact if they butter the BM up when they feel like she has the upper hand. You need to find out more about his feelings about his son... That's more than likely where this is stemming from unless you have reason to believe there's something else going on. Ask questions first, take time to assess the situation.