As parents, it's our resonsibility to make sure our motives are pure when it comes to the kids and our divorce. One golden rule is Do not talk badly about the child's other parent to the children. So I gotta ask, what would be the point in telling your son that his father is a liar and a cheater? Is it going to make a difference somehow? Is it going to improve your son's life and contribute positively to his future development?
As parents there are some things we just don't share with our children, even when we are being honest. Dirty nasty details about divorce certainly seems like one of them. Especially if the kid already has an idea that things are over. If he wants to know more specific details I'm sure he'll ask. Even then, I'm sure his fathers true character will reveal itself in time to his son, without your assistance.