6 joules is not going to kill his goats. It will get their attention and create a memory to stay away from that wire. It will also withstand more weed or grass contact etc.

Yes he has a short somewhere. Should be able to locate the short with something like a Gallagher fence tester/directional fault finder.

I have some family Icelandic sheep out with my cows. After they are shorn the hot wire works for awhile but after the wool grows back a bit they ignore even a 6 joule hot wire due to the insulating effect of their wool.  You need joules of energy storage to get through hair coat. Cows approach a hot wire with their nose and 6 joules works well without being dangerous.