Father's always worry about saying the wrong thing. Just be honest to him, completely honest. I'm sure after all the lies he's been told he'd appreciate some honesty finally. All I know is that he knows his mother is actually alive. He found out a few months ago when he was going through his grandmother's things. Besides that I'm not sure he knows much more, like his mother's new identity and such. It might be best not to inform him of such yet, maybe talk to her and encourage her to tell him herself.