If you haven't already done so, you might dig up a copy of "Three years and a half in the Army, or, History of the Second Colorados" By Mrs. Ellen Williams. It was published in 1885, and is a pretty comprehensive history of the regiment, including its earliest incarnations. It probably won't specifically mention your ancestor, but it might give you an idea of his experience.

And in addition to ordering his military files, you might also see if he has a pension file if you haven't already done so. Pension files are very often filled full of personal information on a subject.