Talk to Neuro Physician on Unexplained Severe Headache With Neurological Symptoms Despite Clear MRI/CT

My father (55) has had a severe left-sided headache around his eyebrow and ear for the past 15 days. The pain becomes much worse when he lies down or tries to sleep, and it improves somewhat when he sits upright. Initially his appetite was normal, but he later developed severe nausea, completely lost his appetite, had episodes of fever, and became extremely weak.

During the past week, he has fainted twice. Along with the headaches, he sometimes experiences confusion, marked weakness, difficulty speaking, and mild trouble hearing. His recent brain CT scan and MRI were reported as clear, and his platelet count is above 110.

Given these symptoms — especially the positional nature of the headache, the fever, fainting episodes, confusion, and speech difficulty — could this still suggest an infection, inflammation of the blood vessels (vasculitis/temporal arteritis), or a cerebrospinal fluid pressure problem even with normal CT/MRI findings? Would further tests such as a lumbar puncture, MRV/CTV, or CTA/MRA of the brain and neck vessels be the appropriate next steps to investigate the cause?