You don't have to tell him when it became official. Just tell him that you felt he needed to know that you and daddy are now divorced. You could just say you know that it is hard for him becaue he wanted you together, but you are also sad that it didn't work out that way. Be as honest as you can without too many details (or name-calling as tempting as it may be.)
"You can't stop the waves, but you can learn to surf." Jon Kabat-Zinn
