The biggest thing to remember is that you don't have to decide tonight. There are no immediate crises, so you have time to sift through it all.
Possible things to consider: How are you feeling with things in general in the marriage? Does your son know that you know what he saw? Have you considered counseling for yourself as you work through it? You know he's drinking, so you pretty much have your answer about how committed he is to recovery at this point. How do YOU feel about this? If you speak with him, what are your expectations of the conversation? Are they about him or about yourself?
My husband started drinking again this year after 18 years sober/dry. I met him when he was a year sober. I'm still REALLY in the confused stage myself. He was sneaking alcohol, then I asked him and he told me point-blank that yes, he was drinking. Since then it's been open, but I know that there has often been more consumed than I've seen. That's slowed down and I'm trying not to get my hopes up. That expectation thing, you know?
I feel for you. It's hard to know what way to go.