Why don't you buy a few pepper spray cans? Each time the dog starts to bark, just give a quick, short spray of the stuff over the fence. I guarantee that even the dumbest dog will make the connection.
Another option might be to ask him if you can look after the dog when he's away. Then you can teach the dog yourself not to bark and maybe even have a nice part-time pet.
Another option might be to ask him if you can look after the dog when he's away. Then you can teach the dog yourself not to bark and maybe even have a nice part-time pet.