You're off to a nice start! One of the keys, to me, in using one light/OCF like this is to try to match the ambient light as much as possible, or the opposite side from your flash gets lost in the darkness, as what has happened here. Doesn't mean it's bad, just not my style. I would have kicked up the ISO, slowed down the shutter speed, and got more light in on his dark side. Of course, a second light could do that, or a reflector, but I don't think you had either of those options. Just my thoughts. Like I said, this is a really nice 1-light OCF shot, and you're off to a great start.
When working with one OCF, a method that frequently works is putting the camera in manual mode with spot metering, and then setting the ISO, f/stop and shutter speed to properly expose the face. Without making any other changes, you would then lower the overall exposure by increasing the shutter speed by one stop. After making these adjustments, the OCF (in TTL mode) should properly expose one side of the face, and the other side will be one stop underexposed.
There are other ways to accomplish the same thing, but this method has been fairly reliable. It appears that the sample image you posted was shot in a very dark area, which would have meant a high ISO, wide aperture and slow shutter speed, perhaps slow enough to need a tripod.
