If the drugs can't enter due to the BBB isn't the only chance to prime his immune system to handle it? Maybe another use case for specialized mRNA vaccines?
They can and do put drugs directly on the other side of the barrier. Intrathecal administration typically uses an infusion pump to put drugs directly into the cerebrospinal fluid. Pain meds are a common use case, as well as some chemotherapy drugs. It's a somewhat risky MOA, as the likely path of infection will proceed immediately to the life threatening stage, which means not many drugs are approved by this route.
The blood brain barrier is the walls of the arteries. Perhaps you can "inject" something in the interstitial fluid but I imagine the volume would be tiny and fluid motion very slow.
