I did nit find a Hiram in Ross' Texas Cavalry Brigade, but the following is a list of all Holt's in the Brigade. Maybe one of them is kin.
HOLT, Alfred, Pvt., Co. B, 3rd TX CAV
HOLT, John M., Co. "H”, QM Sgt., 6th TX CAV
HOLT, Frank, Pvt., Co. B, 9th TX CAV
HOLT, John A., Pvt, Co. B, 27th TX CAV