Well, I know that every state is different, however, I think in Illinois, if a grandparent were going to try to go through the legal system to make any changes to a child's living arrangement without the parent's permission, she would FIRST have to make a custody case and be named the legal guardian of the child. I sincerely hope that your mother would not be awful enough to you to do something like this, but I know that a LOT of east coast states will hold a child's parents responsible for the actions of their children, disorders or not. That being said, if a custody case is required for her to do what she is trying to do, she could try to make a case against your parenting abilities based on your child's serious behavior problems. NOW, while WE KNOW that these behavioral problems our children are plagued with have NOTHING to do with our abilities as a parent, the COURT system doesn't see things the same way we do.
I don't mean to be discouraging, and as I said, I would HOPE that your mother would treat you better than that, but I think it is better to be prepared, just in case. And I am not trying to speak badly of your mom or you... so please don't take offense to that.
Our difficult child's bio mother has tried to bring up our difficult child's behavior problems in the courtroom before as grounds for trying to get custody of her daughter back, but luckily for us, we have our difficult child's therapist and psychiatrist making written and documented statements that while our difficult child DOES have some behavior problems that ARE related to bad parenting, that they were caused by HER abuse of drugs and neglect of her daughter, NOT by our parenting methods since she has been with us. These are the same pros that have been seeing her since BEFORE she came to live with us and they are also making GLOWING reports about how much PROGRESS our difficult child has made since she moved in with us....
Anyway, where I was going with that is, that I hope it doesn't come to that for you, but if it does...you would do best for yourself and your son to ensure that his psychiatrist and therapists and all others involved in his case are willing to stand up for you in court.
Good luck, I know this isn't easy for you and especially coming from someone who should be part of your SUPPORT system. We always expected that difficult child's mother would battle us in court, but coming from your mom, I am sure that it would be quite a slap in the face.