Unless you can find a roster or genealogical information online your best bet might be the regimental descriptive book. Sometimes regimental histories have this as well. You can also search online through the General Orders of the War Department (Google Books has these), which often list officer promotions, at least through 1863. I think that whether or not your persona received a promotion might also depend on the strength of the regiment. If it had lost a great many men, the billet for a full surgeon might also have gone. Quite a few depleted regiments were commanded by captains.
M. A. Schaffner
Midstream Regressive Complainer