Let me first say that, "I am not a parent". Never had a child but I definitely would like to be (In spite of the fact that I am gay!) :p .
I think when and if you approach the subject with your son depends on two major factors...
1. How close the two of you are.
2. Whether you think that he is being abused.
If you feel relatively confident that #2 is not happening then really it is only one.
If the two of you are close (And I suspect this is the case), when he is READY to ask you questions, he will. You are already being open enough of a father that he has seen the books. You're right! (Having been a kid once many, many, many years ago... ok, maybe only many, many
), I can say that children are very observant and pretty smart.
What are your reasons for him wanting to be aware of the abuse that you went through? If it is to help him understand why dad does some things and you want to impart your values of honesty, honor and what a man is and it allows you to be closer to each other then "sounds good". Sit down with him and broach the subject. Be careful not to 'unload' on him. You may not be aware that you are doing it but remember, sometimes 'our' thinking gets a little out of whack.
Broach the subject and let him steer the questions.
That's my two cents.
Like I said, I'm not a parent... yet, but I do know what I wanted more than anything else from my dad then and now.
Love, Respect, Honesty, Guidance and Compassion