Glad to have you here.
Sounds like your boy, no matter his size, will be well loved.
I'm wondering if the pup received any shots or treatments at the vet that might have irritated his stomach? Also, I would recommend that once his tummy settles down, to try a higher grade food than Purina. You might want to check the Yorkie Health & Diet forum on here for some information on high quality foods.
Don't worry about asking questions - this is a great place to look for answers.