What about a leash and (serious) harness? Something that would limit the distance and prevent the pup from actually getting close enough to take a leap(?) I'm not talking about a harness made from straps, but one that would cradle the chest and stomach area enough to gently suspend (him) in the air -- just in case.
Granted, you would have to take it on/off every time your dog goes onto the balcony, but it's the only thing I can think of to really make sure your dog is safe.
