I hope the new vet takes your concern a little more serious. I think all of us understand completely how smitten you are with your little guy. It has happened to all of us or we wouldn't be here
!!! If the shaking isn't related just to fear, I would also be concerned and request blood work be done.
I definitely would not feed him the prescription diet food. It is just as bad as the ProPlan. You may have one who just doesn't like typical kibble dog food. I also would either try home cooking for him or one of the pre-made raw diets. It seems that most picky eaters really begin to gobble those up!!! And, it is definitely true that with premium foods, they require very little because of their size. 1/4 c a day is very normal and it is also a very small amount of food.
Does he eat treats? There are some great, very nutritional treats out there. I personally love Ziwi Peak (you'll most likely need to order it) for my girls!!!! Good luck and keep us informed.