Your claim may not be for medical malpractice but for negligence against the nurse and the hiring agency. Consult a personal injury lawyer in your area. You will need your son's medical records from the injury for the attorney to review.
In Texas, if it sounds like medical negligence then it probably is. I would contact an attorney who does medical negligence cases in Texas. These can be fraught with traps for the unwary, so make sure that your attorney does medical cases routinely. My firm would be happy to review it, if you would like. We handle medical malpractice cases all over the state.