There is a very high probability (depending on the jail, of course) that at least the beginnings of rehab, in the form of Alcoholics Anonymous, are READILY available to him. Also, there will be few distractions that might serve to keep him away from the meetings. After he gets out, he can seek formal inpatient or outpatient treatment if he seriously wants to stop drinking.
Jail is the last place to beat any addiction. If only your rose tinted view was correct.