Before anyone can help you with this, you'll need to get more information on your grandpa's unit. If he was in the Texas Volunteer Cavalry, his unit should have a number in front of it, like 1st or 2nd or such. And his company is probably wrong. I have never heard of any regiment with a Company R. There was Texas cavalry at Yellow Bayou, but there were a lot of Texas cavalry units in the War, and without his regiment there is no way to know if he would have been there or not.

On a separate note, there was an Orrin Morris who died at Yellow Bayou. He was a Yankee.