[Britster]

When he barks at someone like this.... get a squirt bottle and squirt him and say "No bark!" My dad's rescue dog is the same exact way. It works wonders with him. He HATES being squirted in the face. Eventually, you will only have to give him that look and say "No bark" and he should stop.

If squirting doesn't work, get a soda can or a coffee can and fill it with pennies/coins and shake it loudly when he barks and say NO Bark! These work!

Also, make sure he is introduced properly to everyone.
Maybe let your dad/other men get a really yummy treat that he loves, bend down to his level, and give him the treat. Yorkies are so small, sometimes tall men are intimidating to them, so they just need to get on their level.