W. H. Portwood is shown enlisted as a Captain in Company I of the 23rd Texas Cavalry (CS).
Have you considered obtaining his Compiled Military Service Record. You can do so by clicking on the link below or on the ad box at the top center of this form or on the Main Message Index page.
BTW, I will be contacting you by email. Your last name is phonetically identical to a family name in my wife's ancestry Roundsavell and a number of other spellings.