How do I tighten this 10s Shimano splined freehub? 2008 Cannondale CAAD9 R5 - WH-R550

The rear wheel on my Cannondale has been having freehub play for some time now. I’ve tried using an hex key to tighten the freehub, but to no avail. The wheel is a WH-R550 rear wheel.

The 11mm hex key I’ve tried is too big. The 10mm hex key doesn’t catch. I had tried a 10mm hex key on another freehub (from a Giant OCR) and it had worked, but it seems like the splines on this Shimano wheel are preventing the hex key from catching.

Could somebody tell me how I could tighten this? I brought the wheel to a bike shop, Roy’s Cycles in Brooklyn, but the mechanics there didn’t seem like they knew what they were doing and told me to get a new wheel.