Welcome to Yorkie Talk. Sorry to to hear your boy is sick.
Chicken typically makes for smaller, firmer, and less frequent poop. Dehydration will add to that. Likely that he is not constipated.
Slime -- is it clear? This can be a sign of inflammation in the digestive track. Dark color can sometimes be blood.
Did the vet give you a diagnosis? Does you boy have a history of gastro problems, pancreatitis? Long term diet will depend on those things.
Plain chicken and broth are fine for short term.
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