Texas deadly force laws allow a property owner to use deadly force to protect their property against criminal mischief, theft or arson at night.
This case doesn't reallt seem to fit those exceptions; Criminal Trespass is not any of those, and it sounds like the kid was leaving.
To further cloud issues, though, I know from personal experience in two counties that DAs in Texas usually will not except charges of Aggravated Assault or Deadly Conduct for shooting (or shooting at) someone with a pellet or BB gun, even though those weapons are capable of causing serious bodily injury or death. To further cloud things, DAs in Texas will argue that a Peace Officer who shoots someone for pointing a weapon that is KNOWN to be a pellet or BB gun is justifiable deadly force, because the officer was in fear of serious bodily injury, or death.
My guess, there will be a prosecution with no conviction.