Welcome to YT and congrats on your new little adoptee.
For the food, if he was always on table food, you may have to transition him to the food you choose (I would highly recommend researching thoroughly, premium pet foods, and also raw diets for canines before choosing) You could boil plain white chicken breasts and white rice (plain) and mix in a little kibble at a time to entice him to eat. And remember small dogs eat small meals
Also, remember he is in a new place and might be experiencing a little fear and unsure about his whole new world or be missing his other people, which could cause him to not want to eat. But you need to keep him hydrated and some nourishment as these little guys can decline quickly.
He still has all his baby teeth at three years old, this can affect how his jaw formation develops/developed and can defenitely be a source of pain. Hopefully you are having them all removed during his dental. (Be sure to ask your vet about pre -procedure blood work)
as a point of reference, my dog who is 4.7 lbs and is active and healthy, eats approx 1/2 to 3/4 cup per day.
This varies a lot depending on so many factors, weight, proper weight, activity level, health status... etc. You can also look on the bags of kibbles for general references of how many kcals per lb of body weight they should consume (these are only very general references)
Hope some of this helps