I KNOW IN THE STATE OF TX.ANY EMPLOYER CAN FIRE YOU FOR ANYTHING. BUT DON'T THEY HAVE TO FOLLOW THEIR COMPANY ?
Ask the owner to install a wired smoke detector. They are much more reliable. Then you can replace it with one with a hidden camera.
Leaving the freezer door open sounds like just an accident, coulda happened to anybody. Sounds like he knows what he wants to do with his life, which is REALLY good for someone his age. He must have his mind on something else which would explain his forgetful attitude. Try talking to him to see if there are any issues he is battling and encourage him to do good in school if he wants to see his dreams of going to Canada come true. If you do these things and show some genuine concern for him I'm sure he will turn out fine.