sjones summed it up on the weather. You can start the day off with fairly cool (and sometimes damp) weather and end up sweating in the upper 70's to 80's. It looks like the weather this weekend will be pretty mild, but maybe wet.
I suggest you think in terms of a couple of light layers for your upper body. Something that you can easily peel off if needed. Bring along a mesh cap or hat that will ventilate your head.
Also, don't forget leather boots. We have a lot of cactus and any number of things with sharp thorns that will easily penetrate the fabric type boot (making that high-tech Gortex stuff pretty worthless!). If you will be walking much to or from your hunting area, you might also consider some type of snake protection. Something like snake-proof boots, or leggings provide a lot of peace of mind.
Best of luck to you. Hunting South Texas can be awesome and I/m sure that you will enjoy yourself.