In my opinion this is not the way. I would have a very honest discussion with him and his doctors all in one room. I am a bit alarmed that they would have him on "multiple" Benzos. Benzos are HIGHLY addictive if you have an addictive type personality. If you have told them that he may be abusing them, I would be highly suspicious why they would continue to prescribe multiple prescriptions. If they don't take action, I would definitely consider a second opinion in doctors.
That's just my take on it.