Legal Question in Criminal Law in Texas

First, the case is not yours - it is your son's. The work done on the case and the discussion about this work are between your son and his lawyer / investigators. You best not "instruct" the defense lawyer to do anything if you are wanting to help your son. You could alienate the lawyer (who owes nothing to you in the first place.)

Valero stations are independently owned and believe it or not, there are many places that still do not have cameras or even if they do, they do not run them, they do not work, or they do not put tapes in them. They should get the tape if one exists but it may not help if, in fact, everything did happen outside the store.

You could go the the Valero store yourself and simply ask questions about the camera and procedures to determine if it is a possibility that there was ever a tape and whether it still exists. (Many places reuse their tapes.)

If your son is charged with aggravated robbery, the question is not really what your son did or did not have (even if his word could be trusted) but what he lead the complainant to believe. (For example, if he had his hand in his pocket and gestured like he had a gun and demanded money, or said I have a gun and demanded money, the case is still aggravated robbery.)