5-word summary: Otolithic dysfunction r/o labyrinthitis!
Since it seems to be head position-dependent in the pitch plane perhaps (given symptoms exacerbation with reading, etc.), I would also be concerned with a structural or functional utricular pathway (inner ear to midbrain +/- cerebellum) dysfunction causing head position-induced changes in the subjective visual vertical. How was torsion measured, with fundus photography? Theoretically, a cortical lesion in the vestibular cortex (parietal lobe) if appropriately placed (e.g., venous anomalies) could create perceptual tone imbalances vis-a-vis the subjective visual vertical; the head dependence would be atypical but I imagine it is possible. Obviously this could be a vestibular migraine (start migravent)...an important question to ask is whether or not the tilt is sustained or episodic, for the former, I would think for STRUCTURAL utricular localization (?MRI brain skull base w/wo or IAC protocol), or quantitative vestibular testing if available (e.g. vHIT, vOCR, vVEMPS, etc.) to mainly evaluate for signs of peripheral dysfunction. Is there a head tilt? If everything is normal I would follow ?yearly with quantitative cerebellar assessments (VOG, etc.) to evaluate for an evolving cerebellar dysfunction -- his demographics and the tempo of HIS symptoms makes a mitochondrial cytopathy a real possibility (assuming the aforementioned things are normal).
Some additional far-fetched ideas (yet plausible): the first hit of one of the novel autoimmune cerebellar antibody-mediated degeneration (e.g. anti-septin-5 that could cause an MRI negative progressive cerebellar degeneration) OR an early labyrinthine inflammation from a paraneoplastic anti-Kelch-11 antibody disease (most common tumor is usually a testicular seminoma)
comment: An inferior division vestibular nerve dysfunction (posterior canal pathway) or damage to the central projection of these pathways along the MLF, etc would lead to head-position induced torsional nystagmus (assuming symptoms are only present with forward head pitching).
-------------------------------------------