Most Helpful Guy
Objectively, there's nothing wrong with a guy meeting an ex. But your boyfriend isn't in the most objective situation, as he's already broken your trust by chatting inappropriately with an ex. (I'm assuming you're being honest in describing the situation, and not just calling it inappropriate because you didn't like it) I also wonder how you can say he was "found out" doing this and yet you'd still describe him as being "very honest" with you. That doesn't seem consistent.
Basically, you're not preventing him from having platonic friends - he is. He already proved he couldn't have a platonic friend, without letting the texts get inappropriate.
You may just have found a guy who doesn't know how to be a platonic friend to a girl. There are a few like that. The bad news for you, is that such men don't let their partners concerns ever stop them from flirting around with other girls.
If a guy treats his girlfriends *valid* concerns with contempt, it should trigger warning signs all over the place.