1) take away the reward - have friends walk by and when he jumps over the fence immediately (and I mean immediately) turn their back on him and walk away. This is a slow correction, it'll take him time to find there is no reward.
2) Mousetraps. Mousetraps are a wonderful dog training tool. While not outside the realm of possibility I have never heard of a dog hurt by them, and I know lots of trainers who use them. Put mousetraps along the top of the fence, so when he hits one it snaps and scares him. This works great, at least in part because it isn't associated with you or anything you do, it's an act of provenance as far as he's concerned.
I found this "google books result" on the topic
http://tinyurl.com/nswcpj