Support orders cannot be modified retroactivly. If an order was properly made and the time to challange the order has elapsed then it is the obligee's burden to pay or discharge the support obligation. If after the order was made the child lived with him there is ability to argue that the child support obligation was discharged by direct contribution.
Laws differ from state to state.
You need to check the law in that state that issued the order. I do not know of any reason that the military lawyers would get involved. In many instances the involvement of the military is to ensure that the civilians are not being cheated by service people.