I don't know, obviously there is some of the story missing. Still, I think in this situation, you almost have to wait until the kid actually breaks in. Can't just shoot through a window because you think he might.
This would be more clean-cut from a legal perspective if he had waited for them to *attempt* to break in (break a window, start prying a lock, etc) before shooting. The window will be a loss anyway whether it's busted with a hammer or a bullet. Firing through a glass window at someone who is standing close to your house is overreacting. I live on a corner and kids walk through my yard all the time. Sometimes they stop and stand around, but I'm not about to start emptying a magazine on them.
Additionally, when shooting from inside a structure at people who are on the outside, a hit on the first shot is not absolutely necessary. Normally I'm opposed to warning shots, but in this case, you have an incredible amount of cover, and they have none - they also have a lot of room to run. One or two shots into the wall next to the window would probably motivate them out of the area at a high rate of speed, and the rounds should disintegrate if you're using good HPs. You can always say you missed.
Due to the Castle Law this may be a good shoot. Someone doesn't have to actually break into a home in order to protect yourself. However I think there is more that that they haven't said. It wasn't as clear cut as they said or they would have arrested him instead of referring it to the grand jury. It will be interesting to see the outcome of this!
If the kids were just cutting across his property, then how did he assume they were going to break in. I can understand if they were searching around for an open window, or standing around maybe discussing it. When I cut through yards as a teenager, I always got out of the persons yard as quickly as I could.
I walked in many yards at night looking at power lines for trouble source. Power out and spokey sometimes raining hard,I made a lot of noise to let people know I was there. Not drumb banging just a greeting.