BTDT
You need to figure out a way to feed him bad data and let him hang himself with it.
I did this to a sysadmin co-worker once, one of my best BOFH moments. He would ask me how to solve a problem, then hijack the associated call out of my queue, fix it with the solution I just gave him, then close the call, thus taking credit for the fix. One day when he came by, I showed him how to fix a complex issue and to do so, I created a small script in a shared directory and showed him how to run it. Between the time he left my desk and got back to his, I tweaked the script subtly in a manner that would break the target system in a dramatic fashion. Sure enough, he copies the script to the target system, runs it and closes the call before realizing that he's actually shot himself in the foot. Meanwhile, I undo my change and touch(1) the script with an earlier timestamp. When he comes back *****ing "dude, your script didn't work!" I was able to run it on another system perfectly: "I guess
you screwed something up when you ran it!"