You need to be very careful not to turn your son over to your mother unless it is clearly in the child's best interest to do so because you could be opening the door to your mother having rights to file a custody action to retain the child. You should speak to an experienced family law attorney before making any decisions.
My response is based solely on the limited information contained in the question. It is not meant to substitute your attorney's advice.
I agree with my colleague - careful and planned actions appear to be required. I would consult with an attorney prior to doing anything.
This answer is for informational purposes only and does not in anyway constitute and/or form an attorney-client relationship.