Hey OP, I'm in a semi-long distance relationship too, so i get where the anxiety is coming from. I think its a really good sign that your bf shared this encounter with you. He could have very easily skipped telling you about this encounter, but the fact that he did shows he respects your concerns enough to be communicative, and that he emphasized his lack of attraction to his ex. If you feel comfortable, you can voice these fears to him, but preface it with understanding and appreciation. Hopefully both of you will manage to figure what the root of these insecurities is!