You could put up another row of fencing on your side to keep them from sharing a fence.
Or, you could try to teach Seth "no barking". I would use a squirt bottle and when he starts the barking, you say in a stern tone "NO barking!" and squirt him. Let him know that you find his behavior unacceptable. When he stops barking when the dogs are there, say "yes" and give him a treat. This won't be a quick fix, you will have to train this for awhile and be consistent.
Another option is the citronella spray barking collars. They are sensitive and will squirt your dog in the face when he barks. This is basically the same as you training, but you don't have to be there.
One more idea, you could put up an "electric fence" line that runs down that side of the yard to keep him 3 feet off the fence area. That would also take some training on your part until he got the idea of where he's allowed to go.
I would think just the constant barking would drive the neighbors and you nuts. Hopefully, if you get Seth under control, the other dogs will leave him alone.
Time is also on your side---as he gets older, he may not want to play that game anymore.
Good luck.
