Hi, I'd like to help you with your question.
There are two actions you can take to help your son. One, you can confront him about his alcohol use and talk to him about getting treatment. If he had an interlocking device on his car, then I assume he was in some type of trouble with drunk driving before. That means he may have been ordered to receive treatment. Although I am assuming that as well, it means that he either needs to get back into treatment or he needs to start treatment now.
If he did have a therapist before, you can talk to that therapist about trying an intervention and confronting your son about his drinking and his drunk driving. An intervention helps those who are in denial about their drinking. Once he can break through the denial, treatment will be more effective.
Two, if you are aware that he is driving drunk or know someone who is aware of it when it happens, contact the police. Although that may seem harsh, it would help get him back into the legal system where he may be forced to face his repeated DUI's and be forced to get more help. It also increases the chance he will have the interlocking device placed back on his car. At the very least, it may save someone else's life if your son should drink, drive then have an accident.
If you have not done so already, you may want to consider becoming involved in Al Non. Here is a link: http://www.al-anon.alateen.org/. This organization helps families of those who are addicted to alcohol. They offer support groups and resources to help you find the answers you need to help yourself and your son.
I hope this has helped you,