Sometimes pure canned pumpkin helps them go, but it really depends why they are not. So you have tried holding pup, putting a small bite in mouth, closing mouth, and waiting for swallow, then repeat, right?
The vet that mentioned the liver and brain may be onto something. Yorkies are picky and then that can lead to hypoglycemia, but when they are really picky, there is a concern that food makes them feel sick. In that case and with seizures (the ones that they get even when eating), the liver needs to be checked.
I would call the vet back and tell them you still can't get him to eat.
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