Stormtroopers apparently use the same rank system as the Imperial Army, but rarely wear any type of outward insignia. It is mentioned that Stormtroopers most likely have an identification scanner, built into their helmets, which can determine the rank and seniority of other stormtroopers.

Ranks of stormtroopers include the standard trooper, the corporal, the sergeant, the squad leader, and, higher than the rest, the chief. RELATED: Star Wars Sequel Trilogy Connections...

Thanks to costume designers Glyn Dillon and David Crossman, Imperial Officer ranks can be accurately identified by their single-layer badges of colored tiles. Among the most prominent branches of the Imperial Military are the Imperial Army, Navy, Starfighter Corps, and Stormtrooper Corps.

How does the ranking hierarchy of the stormtroopers work? We see they have commanders but how high does the rank go? Are the stormtrooper commanders subservient to even something low rank like Lieutenant officer?

In this system, he'd retain his Stormtrooper Corps rank of Major, but assume a title/position something along the lines of aide-de-camp. He'd still technically be able to command Stormtrooper forces, presumably the Moff's personal guard (see: Moff Kentor Sarne's Stormtroopers).