Hi Cousin Linda,
It's been a while. Hope all is well with you and yours. This man, according to the National Park Service site (see link below), was a Corporal in Company G (Mississippi Volunteers) of the 3rd Battalion, Mississippi Infantry - not Cavalry. This company was recruited from Ponotoc County - part of which was, of course, included in Union County when it was formed in 1870. This unit later, upon expansion, became the 45th Regiment, Mississippi Infantry, and then later still, when faced with apparently irreplaceable loses, was once again designated the 3rd Battalion. While not an original muster roll, you may find a roster (with many duplications and not a few omissions) on the NPS site. Mr. Pitts, through the message board service, will be happy to reproduce his records.