Coupla questions: Is he neutered? Is he going to the same place when he jumps the fence? These are questions you nay need to address to determine why he's going over the fence. If he's intact and there's a "girlfriend" around this will be more difficult. The easiest answer is an anti-jump harness which doesn't allow the dog to get the stretch for take-off that he has without it. Is he actually jumping the fence or climbing it? You might consider putting a mild charge to the fence to keep him off. Nothing to hurt him, just enough to discourage climbing. It's really scary when a large dog like this gets out, because people who don't know that he's more likely to lick them to pieces than attack may call the authorities on him and have him taken away, so make sure his tags are current and on him at all times.
The other thing I would suggest is get him involved in obedience training, agiltiy, flyball (Border Collies are the dog of choice in this sport), frisbee, anything to keep him worn out. You have a dog that probably has more brains than he knows what to do with and is most likely BORED. Is he out all the time? Do you interact with him for more than 15 minutes at a time? Find this way-too-smart dog something to occupy his mind and he should stop doing the Houdini routine. Good luck.