Poor little guy and isn't it a sin on how some people treat their animals...I see it all to often.
Okay here is my advise and I hope it helps.
1. First you need to take this little one to the vet to make sure it does not have any diseases that can be passed on to BooBoo. I would do a blood panel but it is not cheap but well worth finding out what shape your little one is in.
2. The vets office could also do a flea dip that could kill these miserable little buggers.
3. Your little one needs a lot of time to adjust and should be put into a very quiet area and everyone leave him alone for a while until he starts making adjustments and cries to join everyone.
4. He is biting out of fear and could bite you and any member of your family. This is the only defense these little ones have.
When I take in a rescue in this shape first they are kept away from the gang until the vet has cleared them from any disease. Sometimes they stay in the spare bedroom for 2 weeks as I will not harm my other dogs. I then take the new one out to the backyard and put them in an Xpen so they are protected from the other dogs until the doggy HI (smelling each other is over with) and then I let him come out of the area. My backyard is fenced so everyone can run around and I don't worry about someone taking off. It is critical you watch for any signs of aggression from any of the dogs and you will need to step in if there are any issues. Fear aggression is what is going on in your home right now and you need to keep the dogs separated until the new one becomes more comfortable. We call this the honeymoon period.
What is your other dog? How many children are in the home? how many people are in the home?
Quiet and rest is very important during these transition times. No one should force this little one into a situation where he is frightened. Do not introduce him to any other dogs as Yorkies have a HIGH fear level of other dogs and will attack which means the other dog could go after him and kill him. Other dogs might not want to do this but it is their nature to protect themselves.
What are you feeding him? He might end up with diarrhea due to a change in food.
I hope this helps you.
P.S. How much does he weight?