As a dog owner who has had a fair share of incidents with dogs loose and attacking causing me to have researched the law (I am a paralegal so I am trained), I have to say this guy is not justified in shooting on more than one level.
First, as to fencing, mend the fence. Simple as that, you have a history of problems, it is a property line fence, plug the holes. I have bricks laid along a stretch my dog has crawled under, a wood railing at another and have repaired other sections.
Second, the dog bite statute specifically allows for shooting an attacking dog with a specific exception for a dog attacking another dog or a cat. While a lot of leeway in past incidents I am aware of is given to a human walking a dog, that is mainly because how is the human to know who is being attacked. This is a case of just shooting the dog attacking your dog and the statutory protections do not apply.
Grant me the serenity to accept the things I cannot change,
the courage to change the things I can
and the Wisdom to know the difference.
Carrying a gun is a right, not a crime.
Gun control is racist.