[JoeyP]

Hmm ... it sounds like this might be a stressed out little dog
Yorkies can be very vocal but for him to remain barking for so long isn't very good for him
does he shake a lot?

It maybe that he is just scared of people
Why don't you try the intro training

Have somene new come into the house
have them just come into the room
he might bark but don't have the person take notice
let your dog go to them
Give this person a treat for him to hold
he is to leave it in his hand ... let your dog search it out
make it something he loves .. that he'd come running for
once he stops barking and goes for the treat
then the person can give it to him and say GOOD BOY
he'll assoc people with good things instead of being afraid