The mystery continues.
His pension application states... in 1906 stated he enlisted into the 16th Mississippi, commanded by a Johnson and captained by a McAlpin in 1861 (this is in conflict with his headstone application saying he was conscripted). Also states his age as 71, which would put his birth year 1835/36 which makes more sense. He mentions being detailed to "make shoes"
The only Captain McAlpin I can find was in the 46th Miss.