Your alternatives here were not good ones. You could have assigned family and friends to monitor him, which is a job not a party, only to fail. Its clear from your sons response he intended to drink as he stated "everybody drinks" not "I won't" (even though that has been a proven lie and I would have stood my ground).
Your husband's (kind of) non support in the decision is typical codependent behavior in caving into the A at the expense of everyone else. Everybody tip toes around the Alcoholic trying not to upset him, and that's really bad for all parties involved.
You are teaching him there is accountability to actions. Lexie is right, stop hurting yourself over this. If you had let him come and he embarrassed you (which he would have) you would beat yourself up over inviting him.
Enjoy your party sounds like it will be a great time!