Addicted Son

I have a son who is now 18 yrs. old. He is a drug and alcohol addict. For the last three years I have kept him in and out of rehabs. I have done everything I could legally do to help him. Now he's 18 and just 2 months ago he was thrown out of rehab. I let him come home to live and I set limits as to what was expected of him. (curfews, getting a job, going to AA or NA meetings. Etc...) Well needless to say he did not comply. He was arrested a week ago for minor in possesion of alcohol. And just 2 nites ago he disappeared for over 24 hrs. on a drug and alcohol binge!! I am so done as a parent! I know I cannot help him any longer and he has a little brother at home that I do not want influenced any longer. We live in Massachusetts. Am I legally responsible if anything bad happens to my son since I kicked him out. He has not come home to collect his clothes and belongings. He did tell me before he left that if I did this to him (throw him out) He would do something drastic to himself. Any help for my situation would be greatly appreciated.!!