My son has T2 hyperintensities

My 8 year old son developed bilaterateral supranuclear opthalmoplegia about 4 weeks ago. After seeing several opthalmologists he was referred for an MRI. The MRI revealed he had multiple T2 hyperintensities in the midbrain and brainstem. He has also undergone an Spinal Tap, some results back..no abnormalities..still awaiting further results. Specialists have no idea on the cause or the condition, but are treating him with prednisilone. Initially, 3 x iv infusions over 3 days, and now on 25mg for 10 days and then scaling down til his is off them.
My question is...if the inflamation (inflammation) does not reduce after the medication...where do we go next? His eye movemets have not come back to normal as yet either.