Great question.....I would suggest as soon as possible. Reading and learning about slavery in the classroom, especially if his classmates are predominantly white, can be a traumatic experience. At least as a parent you can control the information to your discretion, and help modulate whatever sort of emotional reaction he or she might have.
plus its sugarcoated, lincoln freed the slaves, the underground railroad, george washington carver and peanuts.
I remember for myself my parents made me watch roots as a little kid of like 5 or 6