I used www.civilwardata.com to research all the VA 1st Volunteer units. There were 8, and not a single Montgomery was listed in any of the possible units.

Glenn Land has provided info that describes two soldiers with a similar name. Where have you come up with the possibility that your ancestor served as a drummer boy? Both soldiers are listed as privates, with no additional specific service described. As for "drummer boys" older than 21 or 22; I have a photo of an identified drummer (shown with his drum) and he was 30 at enlistment.