I am glad you are taking him to the vet for a checkup, especially check his teeth, small dogs tend to have lousy teeth. In fact, I would start brushing his teeth once a day with a medium brush and some Petrodex (see chewy.com 6.2 oz tube is the most economical)
YouTube has excellent how to videos
Next, dogs do experience grief and sadness, some more than others. This may be why he has a poor appetite. He has been through a lot and may need several months to adjust to a new home.
See General Guidelines for tips: http://www.homeovet.net/dynamic/php/downloads/dog-c8470f2c75dbe4b683205c3919ee2310/dog_diet_complete.pdf
Don’t free feed. Offer a mix of kibble and canned food or a topper (a spoonful of cooked chopped chicken, scrambled egg) add a splash of water. Feed measured amounts twice a day, have fresh water available, add water to his food if you don’t see him drinking.
Leave the food down for 10-15 minutes, if he doesn’t eat pick up the food and store in the fridg and offer at the next meal time. Something like 7a and 4p. If he doesn’t eat x 72 hours, consult a veterinarian.
He will eat, you have to be patient.
My small breeds do well on Nutrisca (dry and canned)
I find this site informative too http://skeptvet.com/Blog/category/nutrition/
