You have received some very wise advice from DnJ and bttrfly. Please listen to them.
My personal opinion is that right now, he is being very honest with you on how he feels about life, where his head/heart are at and the fact that he wants to put his family before all else. Yes, he is terrified to tell his children about the other child. He is feeling insecure and actually he' afraid that they will judge him harshly for what he's done. It's understandable after all that has gone down during his journey.
I'm glad that he wants to remain friends, but you need to understand that being friends with him is going to require you to step back a bit and allow him to continue on w/his journey and not get caught up in his emotional drama. Yes, he's still out there and still a bit of a mess emotionally. He still has a ways to go and right now, just being a friend is all you can be to him. Keep your expectations very low, do not allow this man to draw you into his drama. As bttrfly mentioned, you can't fix him because you didn't break him. He has to do this on his own. I wouldn't offer any advice to him unless he asks for it. Listen closely for he will tell on himself and you will need to sift through what he says for answers.
Eagle, your xh has a ways to go in his crisis. Try to remember, you can't rush the process. It took him some time to go into crisis, therefore it will take some time for him to come out of it, if he is lucky. Some recover completely, others keep some of their new found quirks and others walk the earth in crisis forever.
Enjoy your trip to Greece! Watch out for pickpockets, but the country is beautiful. I do hope you can relax and just enjoy your time away. Try to keep the focus on you and your children and just let go, let God have your xh for a while.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.
