L.W.
It is AISD's responsiblilty. You need to go higher up on the food chain. Call the Superintendant if necessary. There is legislation in place that ensures that your son will get the services he needs.
They don't want to pay for it and they don't want to send anyone that far out. Too bad. It's not up to them.
My son goes to private school in Richardson but we are PISD. They wouldn't even offer to provide services even though they know and I know they are legally bound to do so. I belive that it is the responsibility or your home school (public, not private).
Don't back down. You are the best advocate for your children.
Good luck!