IMO it depends. If his doctor has him on xanax for a specific purpose and he is taking it as directed then he is sober. If he is taking xanax to get high than he is not sober. My doctor originally put me on Ativan for severe anxiety. I went to treatment and they kept me on Ativan because they said they would have put me on it for withdrawal symptoms anyway. Now that I am past withdrawal they are weaning me off of the Ativan. I guess everyone has an opinion about it. I had my first heart surgery when I was sober and I was on morphine, fentanyl, and percocet while in the hospital and I went home with percocet. These drugs were for legitimate use so I didn't consider myself as using. If I would have kept taking the percocet after the pain was gone just for the high then I would have been using.
Sorry I didn't see in your post he was abusing them. He's not sober IMO. He has just replaced one drug for another.