Maybe he's 'grieving' for his daddy. He knows he's gone but doesn't understand, and possibly fears he's gone forever. I know he's at training, but is there a way for your hubby to talk to him on the phone? Maybe even a recording of his voice would help...a video or something..so he can hear or maybe see him again. Webcam would be great if possible. If he could hear his voice, he might figure out he's just 'missing' from his Yorkie action.
Just a thought... I hope everything works out...I hate to think of him feeling so forlorn...and you as well! Take care!