First of all you are not his Mother hon....
His decisions are his alone to make. "Letting go" is to allow him to make his own decisions (driving drunk) and stop the codi behavior of trying to fix him and make it right. Being supportive is just that ... supporting his decisions.
Example: You walk out of the bar and he wants to drive. You ask for the keys and advise him that you are unwilling to drive with him drunk... he insist and gets in and starts the car.... dont jump on the hood and try to stop him (saw someone do that once) dont fight and argue... just go call a cab.
You are allowing him to make his own mistakes.... and in doing that remember that you have to allow him his own conquenses....
Example: He drives home and gets pulled over and arrested for drunk driving.... You do not go bail him out, you do not find him an attorney, you do not take responsibility for his mistakes.....
Make sense?