I don't have anything specific to add to the great advice you've gotten, but I did want to say, you ARE doing the right thing! Switching to a vet who will, hopefully, listen to you, and keeping him on a diet that works for him.
You may have to worm several times to get rid of the little buggers, and giardia can be hard to detect.
Fletcher can't tolerate any veggies, and something about the processing of kibble makes most of that out for him as well. I've been feeding him raw for almost 6 yrs, and he is doing great. Don't worry about a balanced diet right now. A few weeks on chicken & rice won't hurt him.
good luck with the new vet!