William Augustus Lundy did serve in the CSA, but in the Biographical Rosters of Florida's Confederate and Union Soldiers 1861-1865, he is listed as a refugee (deserter) from the Confederacy.
It's not clear if he actually enlisted in the CSA or just went along with his older brothers who had joined. I haven't found any CSA enlistment papers for him.
He did enlist in the Union Army, 2nd Florida Cavalry, Company C on April 21, 1864 at St Vincent Island, Florida as a musician. He was only 15 years old at the time. His older brother Matthew also enlisted in the 2nd Calvary.
William Lundy left his home in Taylor County after the war, some say because the locals didn't appreciate those who fought on the Union side. He moved to Manatee County and discovered the Fountain of Youth. (I'll tell that story later) He died in 1903 at age 55.
The confusion over William Augustus Lundy being the oldest living confederate veteran came up because another man named William Allen Lundy made the claim in the 1950s. By accident or incompetence many of the news accounts of his claim listed the war record of my ancestor, has fame over the fountain of youth and growing oranges and even his complete name. I'm not sure if William Allen Lundy used my ancestor's background on purpose or not but since my ancestor and his wife were deceased at the time, news accounts attributed the life events for William Augustus Lundy to the other man.
A Joint Resolution of Congress on July 18, 1956 authorized a gold medal to be struck and presented to the only four surviving Civil War veterans, including William Lundy. William Allen Lundy died in Crestview, Florida on September 1, 1957. After his death some questioned his claim to be the oldest CSA veteran and said he was not born until much later.