boyfriend is talking to his ex online?

[Lisa]

My boyfriend and I have been together for over 4 years.we have the occasional argument but otherwise, a good relationship.I recently found out something I don't think I was supposed to know,he has been talking to his ex online,they exchanged phone numbers etc.I had been waiting for him to mention this to me but still hasn't (he had mentioned to me awhile back when a different ex of his was moving back to the area) so why can't he tell me about talking to his ex online.how can I approach him about this w/out starting a fight (he is a very defensive person) should I expect the worst or just leave it alone.I have children who live with us (they are not his kids) I keep going over in my mind what I could have done wrong etc... any advice