Just a few thoughts and observations.
In the 21st Mississippi Infantry, there is no Rockwood. In fact no one with a surname beginning Roc. Surnames skip from Roby to Roddy.
The only Rockwood listed in the 1860 Mississippi census is a W.J. Rockwood, a 35 year old house carpenter, born in New Hampshire, apparently single and living in the town of Panola (now extinct, near present day Batesville) in Panola county.The census page is dated June 25, 1860.
So he is almost certainly not your man.
Tomorrow, I will be able to check the 1860 Mississippi census from another "approach" and will let you know the results.