Yuck.
It sounds like you handled an ugly situation with grace and reason. Don't get bogged down in the "Why would he think this was ok?" kind of questions - they won't get you anywhere.
Alcoholics drink at inconvenient times. A non-alcoholic parent probably would have said, "Nope, kiddo, can't come. I'm not in a good spot to drive." My STBXAH certainly has no concept of his intoxication level - he frequently insists that he's "Just fine" when it's OBVIOUS to anyone that he is slurry, stumbling, and completely intoxicated.
I think the toughest part of the whole situation was your child's disappointment, but your loving attention and concern will go a long way here. Alcoholics disappoint. Your son is lucky to have you there to love him no matter what and to explain to him that none of this is his fault.
Big hugs to you, Shivaya.
Take care.
-TC