BTW - I think it is okay to put some things in writing. My first sentence would probably be

"There isn't enough paper and ink in the world to write an apology long enough or deep enough to make up for what I have done.

I apologize to you, and to the family, for my affair, and all the hurt, anger, pain, and devastation it has brought.

You asked for details of the affair I had, in writing. I offer as much as I can here. I want you to know that once you have read this, you will likely still have questions, and you can come to me and talk with me about what happened. Your mother and I are working on our relationship, and I have given her details, and answered her questions. I want to repair our marriage, and our family. I hope that we can one day have the family and marriage we all desire, and I am working on changes I believe will make that possible. For now, I begin with telling the truth."

Then, list the dates and timeline of the affair for him.
Close with the offer of the counselor or meeting face to face.

And whatever you do, make sure you don't lie to this young man. Also, make sure that you tell him if you have gaps, why those gaps are there - and if you are purposely leaving anything out, tell him exactly the truth as to why (for example, if your wife has asked you to leave something out, then you tell him that truth).

SB