Everything has turned out well for the time being.

The boy's father keeps forgetting that we have joint custody and thought the whole decision of where the boy would live and go to school was all his.
Pick your battles.
And perhaps your weapons, because this was not going to go over easy if the wrong decision was made.

Last week - one week before school started - the dad told the boy that he was still deciding between three choices.

A. The boy lives with his mother (me) and goes to the school that I registered him in over two weeks ago.
B. He lives with his grandmother and goes to the school that I registered him in or
C. He lives with his father, takes classes on line, gets a full time job (because his dad can't afford him) and lives in the garage (because his step-mother is going through something and doesn't want him around)

Now, what kind of high school student is going to be motivated enough to study on line???

So, glad the right choice was made or I would have had to lawyer up.