I agree with Esposa, this is a teaching moment. Or will be in a week or so here.
He knows this stuff goes on, and surely has put 2 and 2 together.
Details aren't necessary. We fathers want to be "strong" for our sons and so we often feel the need to hide that things we perceive as "weak" about ourselves. As fathers we have opportunities to teach and show our sons and daughters that it is ok to show emotions other than anger and happiness. In letting them know that it's ok to show other feelings, we also at the same time get to show them how to handle them.
Don't get rid of your book. What does that accomplish? It won't make the fact that he saw it go away. It won't undo it.
Another question, how do you know he hasn't seen that 700 Club piece from years ago? Your son loves you and nothing will change that.
Recently my son said something that leads me to believe that he thinks I only show emotions when I've had a bit too much to drink.... I know I need to talk to him about that, but fear and awkwardness has kept me from it.
I think I need to fix that and should talk about other things too.
Seize the Day. Maybe don't wait a week if you don't have to. Take him out for a snack or something and just talk. You can do this man, you have been through FAR FAR tougher things than this. Really, you have!