How do you prove a negative? The onus is on the sherriff, not the private citizen, to prove that the citizen does in fact drink. The private citizen doesn't have the burden of proof.
The prior DUIs prove that, and only the fact that he has not been caught for ten years is evidence that he is not drinking. There should be an appeal process for cases like this.
In our county obtaining a carry permit requires that you prove your need to carry to the Sheriff. I have no figures to prove it, but have heard that almost all applications are denied.