Your ancestor served in Col. Leonidas Martin's 5th Texas Partisan Rangers (Cavalry). I have a website for this unit if you'd like to visit.
Hope this helps.
If you'd like to order this man's Compiled Military Service Record, click on the ad at the top of the page or the link below this message.
It appears your ancestor also applied for a Confederate Pension as William R. Jennings from Grayson County, TX. You can order this information, as well.