Do you have insurance for him?
If so perhaps you should see if it will cover some further investigations. It sounds so distressing for you and must be horrible for him.
I wonder if it is an allergy that could be treated with homeopathy? Some insurance companies will pay for some limited 'alternative' medicine. I think the vet has to refer though. I would go back to your vet and demand some answers.

Let us know how it goes.

Laura