Hey thanks, mac!
I'm guessing that the army uses the term in the same way that the navy does.
Regarding the practice of frocking itself, there are various instances in Navy Regulations at least as early as 1802 of personnel assuming the uniform of the next higher rank, not necessarily with higher pay, when appointed by proper authority to assume the duties and responsibilities of that rank prior to actual promotion.
This, in particular sounds like his situation. They don't let him wear the uniform but he has assumed the duties without the actual promotion.
It sounds like the word deals with the clergy too - I wonder if they call it getting "frocked" when they receive their orders (is that the right term?)?