sorry I didn't read your follow up related to the love letter... but my opinion remains mostly the same. It's understandable that it would bring up more suspicions in you. That being said, I think that still at this point nothing he's said brings up red flags for me about him. Maybe about his ex, but she could be determined to get him back even as he keeps ignoring her. Some people will try even harder when their target person ignores them.

Even though she may be the one pursuing him and he's ignoring her, I think for your sake, if it were me, I would cut off all communication and unfriend her. Not saying he is doing anything wrong per se, but that it's always best to eliminate (if possible) anything that's feeding the insecurity of the other, if it's in your power to. As a man, I just think he would be doing the right thing to do so. Whether or not you should ask him to overtly is another question. I'm not sure if I would, since at this point, he hasn't shown any real evidence of really communicating with her. At the very least, though, I would bring it up, not so much about his past, but his contact with her. Keep the focus on your feelings and let him know it just makes you uncomfortable, without telling him to do anything, explain why but don't probe too deeply about what they were together or anything like that. Just talk about how you feel and then leave the ball in his court at that point.