I'm so sorry to hear this. Reading your story brought back lots of painful memories. We lost DSS to a move away several years ago and it was awful. She moved him two states away, a 2 hour plan trip or 18 hours in the car.
DH fought the move, and the court experience was horrid. Despite the fact that he had been an involved father all along (had his son 35-40% of the time, never late on one cs payment EVER, happily paid for extras at biomoms request), DSS is a biracial child with very serious (life threatening) special needs & developmental delays, the judge put the onus on DH to "prove that it would be detrimental to the child to be moved away from his father". WHAT????? The courts do not value fathers AT ALL and I think it's absolutely despicable.
But I do want to offer a bit of hope. Our biomom's big move didn't work out as well as she thought and 2 years after moving away, she moved back. And DSS is actually living with us now and he sees his mom 4 days a month (alternate weekends). In fact, we now totally have the upper hand because she does NOT want to go to court again. We live in CA, and once you are assigned a judge to your case, you always see that same judge. So the same judge who granted her move-away 5 years ago would not be happy to see her back in court, with her son living with his father, the same man she tried so hard to make look like an unimportant part of their son's life. Oh, and why did she move back? She met a man while she was her on visitation weekends here in the area (special needs child could not be put on flights alone - she or DH accompanied him on each visit) and they started dating, so she decided to move back. Yeaaaahhhhhhh. Real stable, lady.
Anyway, you just never know what the future holds. I hope things get better for you all. In the meantime, hold strong and do your best to make the time you do have with DSD special.
Again, I'm so sorry.