I know it doesn't feel this way, but this is an opportunity to grow close to your son. there is NO need to share details. no reason to tell anything really beyond what he already knows. But if you can find it in yourself to be open with him you can grow closer through this. I am speaking from my experiences with my own kids. once we were able to have a more direct communication (they still don't know everything and may never) I found out how frustrated they had been. They knew all along that I had something there,they just didn't know what it was. It was in many ways freeing for us all. The most important thing is to just let him take the lead. just bring up the book and say that you noticed him reading it and let the convo go at his pace. I know you can do this man.
Towards thee I roll, thou all-destroying but unconquering whale; to the last I grapple with thee; from hell's heart I stab at thee; for hate's sake I spit my last breath at thee. let me then tow to pieces, while still chasing thee, thou damned whale! Thus, I give up the spear!"