Your on-camera flash can be turned down to just light the guy enough, letting the rim light shine still. Its a Flash Exposure Compensation setting as shown here in the photo, center icon, and is controled by the wheel under your index finger when camera-is in-hand:
On the Fuji S5 Pro, it will be in the set-up menu #1. Bring it up, highlight it, scroll for a reduced value, and shoot. The camera will trip the SB800 and you'll be fine.
Here's another interesting thing to try: Put your camera in manual, pop it on a tripod, set for ISO 100, 8 seconds at f/22 [if you can go to f/22, if not wiggle the exposure around] and, having your subject remain dead still, take your SB800 in hand, walk to about 45° of him and pop it once, walk around to his back side on the opposite side, again 45 ° off and pop that flash again.
So, ISO 100, 8 seconds, f/22, 1 pop full power at 45° front left, 1 pop full power at 45° rear right. Make sure he stays absolutely still between the two pops. =) Its kinda cool. If he moves, youve just wasted 8 seconds =)