Uh...is he challenging him from the left? Because I don't think you can get to the right of him.
2014 saw some to the left GOP challenges (See: Tim Huelskamp, Chuck Fleischmann), so I think it's probably that.
Republican city mayors tend to be the closest to moderate that you can get in Texas. They have to deal with the problems the federal and state governments pass the buck on. There's no room for them to take the conservative line of, "We don't need to spend money to fix this problem. Let the [lower level of government] deal with it."
It sounds like this guy's main priority is championing agriculture and the farmers in his district, while also being generally socially and fiscally conservative. In other words - if he were running 30 years ago, he'd have a "D" after his name.