Unfortunately, in this case LE is not going to go out of their way to solve this case. This is a very small town. Very close knit people. If Brandon were a part of one of the prominent families they would never stop looking. I think the sheriff's office believes he was running (or drug activity) and he is not worthy of their time or money. The only hope here is the Texas Rangers and I am not sure they have enough evidence to investigate a lot.
As for being out on the flat land, there are plenty of hills and pastures to hide a body. It really isn't flat at all here. If he got lost, he was close to the highway with lighted houses in close proximity. No way he died from the elements.
Brandon "ran into some guys". I think they were unknown to him, they got into an argument and Brandon lost his life. They loaded his body up and took it back in the direction of San Angelo and dumped it somewhere. May be someday someone will run upon his remains and there will be answers to what happened, but I doubt this case will ever be solved.
Sad for his family. I too do not suspect family either rascal, but I can respect others feelings about why they do. I feel they have been evasive because that is their way of life with LE. When they have been involved in illegal activity, they can't feel trustful of LE. That is why I think the family looks suspicious.