Just a thought .... my son's friends are always borrowing his cell phone to text others...including their girlfriends ... because they don't have a phone themselves or their batteries went dead. Also...using someone else's phone would be a great way to conceal questionable conversations from their own wives or girlfriends?!
I don't know if this could happen in your situation ... but just thought I would bring up the possibility that maybe those weren't his text messages ... but one of his friends.