First, please make sure that your prank victim is not carrying a firearm.
Once he is sound asleep, unstake the front of his tent and make sure that the door is blocked with sagging tent fabric. If your prank victim is not the type to panic, alternatively, you could use string to tie together the tent's front zipper pulls.
Then, using a very high power spotlight (car headlight brighness) start lighting up his tent with momentary blasts. Blast the light just enough to make him stir, but not come completely awake. Next play your ghost wails sound track in short intervals.
Then when your victim is awake, run your own tent's zipper pulls up and down rapidly to simulate opening a tent and run around in a disheleved manner asking loudly WTFWT.
This trick was actually pulled on me at our local Two Sisters haunted camp site. In planning the trip, the guy with the idea of camping there had told some stories of the camp site being supposedly haunted. ( Our trip planning was accomplished at a pub over a few adult beverages.) Then on the way down to the campsite, someone mentioned that they hoped the site was not really haunted. Our prankster trip sponsor laughed and asked if we really believed in ghosts.
Overall, the prank execution was almost perfect.
[I got even the following New Year's Eve when our prankster joined us at a local state park for our regular Escape Party. The prankster did not have a winter weight sleeping bag that could handle the 10*F lows, but since the state park had electric hookups he brought a ceramic heater to use in his tent. Once he was sleeping peacefully, I unplugged his heater. When he started tossing and turning, I would plug in the heater. This went off and on for about two hours. Finally when the champange was gone, my girl friend de jour started giggling and woke everyone up.]
Cheers and remember that paybacks are a biotch.