How does he justify this?
I agree that he's in the wrong. He's trying to justify under the Texas General Code 411.172:
(a) A person is eligible for a license to carry a concealed handgun if the person:
(7) is not incapable of exercising sound judgment with respect to the proper use and storage of a handgun;
but it sure smacks of discriminating for reasons other than a genuine concern under the above statute.
Heck, by his reasoning, anyone who had ever done anything stupid (hasn't everyone?) would be disqualified.

What is the point of rules that are not enforced?