George --

No reason why Boykin couldn't be a volunteer aide, but we have no real paper trail. Plus, volunteer aides became scarce after the Conscription Act of April 16, 1862. (Note that Boykin expresses concern about being conscripted.)

If he really was an unpaid volunteer aide, Boykin wasn't content with that. Otherwise there would be no application for a paid staff position. Also, it doesn't look as if Boykin went to the Army of Tennessee with General Cantey's command.

He doesn't mention a preference for staff service of one kind or another (quartermaster - commissary - pay - inspector-general - transportation &c). A normal application would mention a specific arm of service. Also, Boykin doesn't mention service to a particular general officer. If Cantey was pleased with Boykin's services, why isn't there a recommendation from him?