My son is 10. I want to tell him the the truth but I pray everyday that he figures it out on his own.
You don't need to say anything.
Every moment you spend with him brings him closer to his own conclusions. If he grows up and decides that his mom is a piece of work it will be because of what he's seen and compared over the years. Not only is that far more powerful than anything you could tell him, it leaves you completely blameless.
It's like that scene in Malcolm x where Elijah Muhammad pours ink into a glass of water. You don't have to tell him how dirty she is. Just let him see how clean you are.