Is it a handlebar light or helmet light? If it's a helmet light, try not to look right at the person's face because that shines the light right in his eyes. But if it isn't even dusk yet it shouldn't be blinding him.

What's his attitude? Does he sound informative -- like maybe you don't know you have your light on? Or does he sound like he's mad at you and your light is bothering him?

As far as I know, there's no cycling social discrace in riding with your light on at dusk...