That would be 7th Miss. Cav. Regt. There is also a 7th Regt. Miss Inf. anda 7th Battalion. he is shown in the CWS&S at http://www.itd.nps.gov/cwss/soldiers.cfm this way---
Thomas Ford (First_Last)
Regiment Name 7 Mississippi Cavalry
Soldier's Rank_In 1 Lieutenant
Soldier's Rank_Out Captain
For a unit history go to http://www.mississippiscv.org/mississippi_history.htm
See 7th Miss. Cav. Regt and 1st Regiment, Mississippi Partisan Rangers.