[Lauri]

I hope I don't hurt any feelings since I am new here, but I wanted to comment.
I am only commenting here because as a member of a Yorkie Rescue I have my own ideas about the whole thing. These little guys and gal furbabies are notorious for escaping. In fact next to all the kisses that is their second most favorite pastime in the world. I'm thinking this. Perhaps the reason for his skin and bones attire is the fact that he escaped by accident and then lost his way home resulting in his poor weight when you found him. Many people who own Yorkies do not microchip them since the procedure is rather painful for the dog unless it's done under anesthesia by the vet while getting something else done, like spay/neuter or dental work. Also microchipping has really just become something that people opt to do, and some of the older yorkies wouldn't have had the opportunity as pups to have it done if not done by the breeder.
If I were you, I would in all fairness to someone else that may have loved him as much and more than you do already in your short time of having him, place an ad in bold in the lost pet section of the most popular paper in your area. I would run it for 1 week. I would also request that he be identified by some certain mark that he might have since unscrupulous people would love to have a "free" yorkie. That is just in case there actually is a grieving family somewhere out there, especially with children that may love this pet dearly and lets face it no matter how much you love and tend to these little furballs they can and will escape!!!
If nobody answers the ad. Well then he is yours to keep and you have done everything in your power to make sure he is where he is supposed to or was intended to be. This is just my own feelings on the subject. I'm glad if nobody answers the ad, that you took him in, since as a foster and a member of a rescue organization we see many that are just thrown away or kicked out of the home for one reason or another and wind up in kill shelters etc.
Let your conscience be your guide, so your heart will be at peace with your decision.