that would depend on his rank or just 'sir'. Nothing in the question implies addressing or TALKING about an officer nor WHAT army is involved...........
It also depends on whether you are addressing him, or talking about him.
In the British army, officers are addressed as Sir, but referred to as Ruperts.
There are many different titles for a 'middle ranked' army officer depending on the time period and the country the officer is in. Check out the related links for more information.
quartermaster
The Commanding Officer .
quartermaster
centavase
You would probably be referring to the Army Chief of Staff.
John C. Fremont
Officer in the Army. has written: 'Killarny. A poem. By an officer in the army'
A high ranking army officer would have an enlisted aide to help them focus on their duties and not on things like cleaning, cooking, and pressing uniforms.
Field Marshal Sham Maneksaw
who was the us officer that rejoined the union army
Allan Wilson - army officer - was born in 1856.
Andrew Holmes - army officer - died in 1814.