Willow, he doesn't need to tell you why he hid away. His marriage broke up after a few months. What an absolutely devastating thing to have happen. You do not know what went on in that marriage that led to its early demise. Whatever it was, you step son just wanted to run away and hide. He didn't want to have to discuss it with anyone, he may have felt ashamed and a failure and unable to face the world he knew after such a disaster. I can well see myself doing something like that in his situation.
The other difficulty is having hidden away, coming back is equally difficult, especially as he has rebuilt his life and has a new partner. I think you should feel compassion and kindness to a son, who got lost and then found himself.
He could have walked away and never returned, instead after a period in the wilderness, he realised that what meant more to him than anything else was his family.