Trust me. Little boys love the story of King David - at least the early years. Quick moving and gripping action. He was a man of God and a manly man's man. A true confident leader who was at the tip of the spear. Think "Lord of the Rings" type of hero. Of course, your son will have to get older to understand the great sin of King David. The story of Salvation History takes a turn for the worse at this point and King David is never quite the same. But we did get Psalm 51 out of it!
In fairness to little girls everywhere, the short book of Ruth tends to be their cup of tea. I would subtitle it "The Romance of Redemption". The original Cinderella story.