One of our cats suffers from severe itchiness too. We tried steroids, changing his food, Benadryl, Zyrtec, and flea treatment, none of which worked. What did work for him is Gabapentin. It acts as a nerve blocker and stops his itching.

He also suffers from dry skin which might be a root cause, so we are going to add a fish oil supplement to see if that helps as well.