It's time for
your vet to do some more investigative work. It was reasonable to suspect a drug interaction, but that apparently isn't the problem. It's time to do some imaging studies like x-ray or an MRI to see what, if anything, is happening in the brain. This is particularly important given your information about his recent fall.
Your vet may want to look into possible problems in other areas of the body as well, so expect some blood and urine work.