Thankfully you took this pup in and are caring for him instead of him being out in the streets scared and hungry.
Have you contacted your local humane society, rescue groups or Dog wardens ?
That is always the first place that I would check and I assume anyone who has a missing pup would check. Also, if your vets office has a FB account they will post the information there for you to help in your search. Well at least mine does.
FB is such a world wide venue and can spread fast !