Yep, I've been there too. Typically with me, I'll feel safe enough to put something out there, and then after a little time has passed the panic and fear sets in. After all, this was stuff we were never ever supposed to tell anyone about. It's scary. So he's following a pretty normal pattern, at least from my experience.
Part of him knows it needs to come up, he might not know that consciously, but it's there, so when he's feeling safe and secure, it'll start to come out, but that in itself creates anxiety and fear. I can't tell you the number of times I would share something with my wife, and then not too long after wish I could hide it all away again. That if very very normal at this stage of the game.
You're right, it is a big deal. The best thing my wife did for me was to just love me no matter what, and hold me close and just let me know that no matter what, she wasn't going to leave me. Being validated and reassured is a huge thing right now. He's so extremely vulnerable. After all, you now know some of his "dirty secret".