Thank you for the additional information.
You mentioned vet, groomer, and also boarder.
How often and for what span is he at the boarder?
Also, has Toby been fixed? (I should have asked that earlier.)
Does Toby respond well to your commands--sit, stay, etc.
Socialization is a process of slowly/over time (not all at once) getting Toby comfortable in a diverse set of circumstances--places, people, animals, smells, noises, textures, etc.
My late senior sweetie was the perfect yorkie on our walks--didn't matter where we were walking. Ditto, when she was riding in a shoulder carrier. Our vet thought she was overly protective of me, and overly spoiled by me. She was the ideal patient for him ... as long as I was not in the room.
Here's an article on YorkieInfoCenter.com, "
Socializing a Yorkshire Terrier." That article covers a wide range of issues. The article may give you some ideas. If you describe the situation to your vet, he or she may also have some good ideas.