* He's naturally dark-haired: He's been known for his dark brown hair throughout his career.
* He's aged gracefully: As he's gotten older, his hair has naturally turned a bit lighter and more gray.
* He keeps it low-maintenance: He's said in interviews that he doesn't want to spend a lot of time on his appearance, so it's possible he prefers to let his hair age naturally.
Ultimately, we can't know for certain if he dyes his hair. It's possible he uses a subtle touch of dye to enhance his natural color, but there's no official confirmation from him.