I think he's getting overexposure because the camera enforces the X-sync speed instead of the metered speed.
lizzy -
Get either AF360 or AF540 for the so-called high speed sync (FP), alternatively, as others have suggested, get an ND filter.
In that case, the whole scene will be overexposed. If most of the scene is correctly exposed and the subject is overexposed, it is likely the flash exposure is off.
BC