Getting a DUI is definitely alcohol abuse so there is something there! I don't think you can just overlook that. Not everyone gets DUI's and to me it's either horrible luck or possibly a sign that he is drinking too much/too often? Just wondering?
Self medicating maybe? Many adults do that also.
I will say from experience that if you threaten anything, you had better back it up or your words will be meaningless.
If you do not already, I would strongly recommend seeing a therapist for yourself so that you can maintain your sanity and create healthy boundaries with your son so he does not RULE your life and the lives of everyone in your house. That is no way to live obviously by having an 18 year old call the shots and I know you are trying to get a handle on it. That is what we did but the first warning of the therapist was to make sure you think about what you are saying and that you can follow through; do what you say you are going to do if they do not comply.
As many have said on this forum and it is so true, nothing changes if nothing changes. If he won't change (they seldom do when we want them to) then you have to change. This is tough stuff.