I have to disagree with the last couple, if he gets the rank and the responsibility then you should get the pay, the fact it's an ooa is neither here nor there.
.....but if it helps get him promoted he may very well be getting the pay full time. There are times when I was working with the Army I would have given anything to be A/Cpl unpaid just so that they Barmy boys didn't treat me like one of their clueless privates. You have to think of the bigger picture it may well be of a benefit to him to A/Cpl unpaid.