My very first thought is not how to control the itching, but what's causing it.
When he had fleas, etc. it made sense. But, that's done with, right?
Could it be the food change? It seems to me, unless I'm mistaken, to have gotten worse since the change.
Benadryl can really help when something crops up, but the vet's suggestion on amounts is something I've heard nowhere else. A Benadryl before bed, or 1/2 twice a day, shouldn't be a problem, but obviously, even the 2 a day is too much for him.
It's not only weight, it's how individuals react, as well.
I'll be glad when you can find a new vet, but understand your situation, and how you have to make do at the moment.
Please keep us posted as you make changes.
Love All Life, Thank You For Posting! :)