Hello ! I read your question and understand your concern. In my opinion the troubles he has may be related to his inner ear. The hearing loss is very typical of this . As you mention in your question , he has done several tests. I am taking into account that the brain MRI has resulted normal, because a tumor of the right pontocerebellary angle would give similar symptomes. Diabetes can cause cranial nerve neuropathy. The impaired vision may be related to cataract. I would recommend an ophthalmologist consult to examine better the problem. If the problem is outside the eye, but in the optic nerve visual evoked potentials would be necessary to locate the problem. The acustic and vestibular nerve can be affected by this disorder.I would recommend brainstem auditory evoked response, which would confirm the diagnosis. The numbness in half of the head may be related to the trigeminal nerve disorder, like the two other nerves mentioned above. Hope to have been helpful! Greetings, Dr. Abaz Quka Hope to have been helpful.