You're doing the right things. You do need to be sure the parasites are gone, and you do need to put him back on the bland diet until you find something that works for him--or you can keep him there and add supplements. As mcsheltie said, the longer his system is out of whack, the longer it will take to recover.
And, if I were you, I'd definitely be looking for another vet clinic. It's not unusual for different vets to have different opinions--even when they practice together--but they're not responding to you or communicating with each other, and that's not right.