#1. Dude was asked to leave the property, should've been over right there and then.
#2. Shooter has the right to be armed under any circumstances on his own property, whether or not there's a situation that could escalate.
#3. Shooter is armed and threat turns aggressor, in what world would we not consider it basic logic that if I'm clearly armed and you decide to try to assault me anyways that you're clearly a deadly threat because you're ignoring or trying to supersede a deadly defense. I can see where an argument might be made for the first shot into the ground, but it's not a compelling argument. Also, I'm not inclined to defend a 'warning shot' because I don't believe in using them.
Considering those three facts along with the potential threat to the aggressor's ex, I'd call it justified. The moral of this story is that trespassing is the wrong hill to die on when you're trying to settle a custody dispute. Get off my property needs to mean something.