J.O. ,

It would not have been the 1st Texas Infantry Regiment , as they fought with R.E.Lee in the Army of Northern Virginia for the entire war ( except the short trip to Tenn . they made as part of Longstreet's Corps). Also, considering the area ( the Rio Grande area) , it most likely was not infantry . I do know that Buchel's 1st Texas Cavalry had one company fight at Los Patricios . My guess is (without more looking) , that your relative was either part of this company of 1st Texas Cavalry troopers who fought at Los Patricios or he belonged to another group of State Troops that partrolled the border , or perhaps one of the regiments of the Arizona Brigade."Rip" Ford and Santos Benavides did most of the "organized" CSA fighting , skirmishing and patrolling along the border area , although , at the beginning of the war, some Confederate Partisan troops were engaged in border activities. I have a book at home that goes into some detail about border activities during the war. I'll check it tonight , because , right now , the foregoing info is off the top of my head.
Regards, CAM