Yes, I experienced that w/ Marcel after he had Giardia - and when he had small intestine problems following that. It could be related to what you're feeding - what specifically are you giving for food and treats? He is trembling bc he is probably in quite a bit of digestive pain. There are a lot of folks who can help on this board and we also know of great canine nutritionists!!
Also though, the vets should do blood work on this baby - have they in the past?