How old is your child? I would definately get copies of his medical records to show you are the one that takes care of his needs, takes him to the doctor, and to make it clear that keeping with his current medical providers is definately in his best interest.
I don't know much about autism, but isn't big changes really hard on them? Have you talked to your son? Has she been allowing you contact? If not you really need to be keeping a log and noting all the issues that come up and how she is interfering in your relationship with your son.
When is the hearing? Do you have an attorney? If not I would talk to one ASAP about getting an emergency order for her to return the child to this no later than the first hearing.