Thanks for using the forum. I am happy to address your questions, and my answer will be based on the information you provided here. Please make sure you recognize that this forum is for educational purposes only, and it does not substitute for a formal office visit with a doctor.
Without the ability to examine you and obtain a history, I can not tell you what the exact cause of the symptoms is. However I will try to provide you with some useful information.
MRIs do not involve exposure to radiation, the entire MRI image is generated from changes in magnetic field, and these magnetic exposures are not considered harmful unless a person has hardware in their body such as a pacemaker. Therefore, the harmful effects of radiation seen with other imaging modalities such as CT scan and X-ray are not present.
Your son's symptoms could be due to a variety of things, but obviously one concern when involuntary movements of a limb occur are seizures. However, other possibilities include paroxysmal dyskinesias (a type of movement disorder that occurs with action or emotion), dystonia, choreathetosis (all types of abnormal movements that can be episodic, and have several causes including genetic, post-infectious, and others). The exact appearance of the movements will help the neurologist determine what type of movement it is and the potential causes. If these movements do not occur frequently, such that they may be absent at the neurologist's office, videotaping them and taking the video with you to the neurologist is a good idea.
Followup with your physicians is recommended.
Thank you for this opportunity to answer your questions, I hope you find the information I have provided useful, good luck.
Thank you for your information.