Greetings Andy welcome to the Forum
How wonderful it is to hear, of how our little ones respond to a perceived threat, how much he must love you, to go up against the most vicious and cunning animal on the Planet in your defense. That being the Human Being.
Most Yorkie Males desire/have a Pack mentality, and being a Male, it is in their nature to step forward when a threat is perceived. That duty is demonstrated first by the Pack Leader. So, it seems as though your little guy has not understood, that duty is yours, not his.
Maybe I should turn that around and say, that you have not convinced him that you're the Pack Leader, and in a Yorkie Pack, there is only one.
Punitive measures like the spray or getting "others" to correct your animal work ok, but it doesn't address the basic problem, of him not following YOUR verbal instructions. That's where you need to focus your attention, directly.
You need to start working with this animal, sit, lay down, other stuff, and a basic obedience class, would help a lot. The idea being, that you take control of his actions, not allow him to act on his idea of what needs to be done. Yorkies were bread to function in the absence of instructions from us, and that's what he's doing.
Your objective, is to re-direct his source of information, from his opinion, to yours.