How do you put your child at ease during a custody battle?

My son is an amazing 13 1/2 yo high functioning autistic child. He and his younger sister currently spend the school year with his mom, and summers, weekends and holidays with me. He's made it clear he wants to live with me, and I tried numerous times before filing to talk to his mom about avoiding a court battle he shouldn't have to be in the middle of. But, in January I had no choice but to file the custody motion to resolve before this upcoming school year.

We've had court ordered Mediation that was a waste, but we did won a motion for my son to speak to the judge where he's expressed his wishes to move here. Hes starting to feel worried it isn't happening because he has to go back to his mom in a week until we either come to a conclusion or the judge modifies custody in September. Gotta love the speed of courts. I just don't know what to say to him to ease his mind right now. My exes family is very overbearing, controlling, and has told him no way at the mention of wanting to live here.

For those of you who have gone through this what advice or words of wisdom do you have to give him ease of mind? I tell him I am not going anywhere no matter what happens. It's tough telling mom you want to live with dad, but she wont listen or care as she can't see past herself.