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 your doctor.
Without the ability to examine your son and obtain a history, I can not tell you what the exact cause of his symptoms is. However I will try to provide you with some useful information.
A sensation of brain shaking is an unusual symptom description; sometimes this could be a dizziness or vertigo that a younger person may describe as head shaking? It could also be a term perhaps used to describe a headache. Headaches in adolescents are not uncommon and could be due to a secondary cause (such as a tumor or vascular malformation); a normal CT scan does certainly make these less likely, though in some cases an MRI of the brain and other brain imaging is needed to exclude other causes. In this age group, the most common cause of headaches are migraine headaches or tension type headaches. Migraines cause throbbing pain, nausea, dizziness, and usually last a few hours. They can certainly occur in adolescents. The vision symptoms could occur with migraines, though an opthalmologic cause would need to be excluded as well.
It may benefit your son to be evaluated by a pediatric neurologist. An eye evaluation is warranted too for his intermittent vision loss symptoms, and if on further questioning he reports vertigo (room-spinning), evaluation by an ENT may be helpful as well.
Thank you for using the forum I hope you find this information useful good luck.