You need to,work out a payment plan with the HS. Your son attended, and money is owed. Of your son lived with you, really, you could have withdrawn him at any time and enrolled him in the public school…when you realized you were paying the full bill. HS is four years long…surely you saw the handwriting on the wall,after one year.
Go to the school…and see what you can do.
In the meantime, your son can take courses at a community college…no HS degree required.
He could also take the GED test.