A reflector would help with that dark side. You can get the Chinese collapsiible reflectors very cheap, or pay a bit more for a name brand like Lastolite, etc.... You can also replace or supplement your store-bought reflectors with a simple sheet of foil-wrapped insulation board from Home Depot. If you have a reflector at 45 degrees of the subject on both sides, and you bounce flash off the ceiling, you might get better results with your single flash.
I assume the camera chose your 1/50th shutter speed at 125mm.? That might be a little slow for getting total sharpness in the eyes. (even worse if using the 7D instead of the 6D) You have an ETTL speedlite, so you could go Manual and select a faster shutter and the flash will compensate for the underexposure. If things are still coming out too dark, you can jack up the Flash Exposure Compensation a stop or so. Your lens does have good image stabilization, but I am not sure you really need to be counting on it. Also, IS won't help with any subject motion blur you might get from your relatively still subject moving a little.
It seems like you used the center AF point with the camera in portrait orientation. His face is right in the middle of the shot, but it gave you too much negative space above his head. You can select a different AF point so that your focus point is nearer the top of your viewfinder, and then you don't have to place his face right in the middle of the shot.
Taking it to the next level, you can get 1 or more additional speedlites, possibly some wireless flash triggers, light stands, shoot-thru umbrellas, etc...