Eh, his getup (cargo pants - possibly of the "tactical" variety, like 5.11s, and a black tshirt) is pretty typical of modern "tactical" style. Or maybe casual tactical might be more accurate. Just watch any gun video on youtube, and chances are, you'll see a person dressed like that. True, he isn't wearing anything like body armor or other military-style equipment, but how many of the modern mass shooters have? I'm kind of asking seriously here, I really don't know - as far as I know, they're mostly disaffected young men, who I assume basically dressed like that during their attacks. Either way, I don't think his choice of clothing really matters much.
The gun is totally an AK varient from that picture, though. Kind of strange to see that in attacks two weeks in a row when ARs were so common beforehand. I'm not sure if there's any significance.
Let's face it: balaclavas, webbing gear and such might look menacing but what's the point of dressing up like that if you are just going sweaty and tired?
A tshirt and shorts is probably more than appropriate apparel if you are going to play paint-ball or go on a killing spree in Texas.