That is so great he made that decision. He kinda came home the same way mine did. Pulled up, came in. I am sure your going to feel mixed emotions. Your wanting to know will drive you crazy. Hopefully he will volunteer the information in time. Right now it will probably be withdrawal. He will probably let you know how awful she is and that. But I am sure it had something to do with him. Maybe he found that out in a way. He is the one running. Your not. There is a way to do this so he can open his eyes for himself. Try not to over explain yourself. I mean on the "you didn't care about me" comment from him. Maybe look within yourself and think of how he derived that from. You may of did things one way thinking it showed you care and he is looking in other ways.
Anyway, take it slow and use action. Let him volunteer to talk it out. Don't be eager as yet. I really liked how you greeted him. It made me laugh.
Maybe you could meet him for lunch and have a lunch date. Something out of the ordinary to let him know your his friend and you want to help him to get over this person.
And the alcohol issue, they kinda have to do that on their own for their own selves. The more an issue is made, the more they seem to crave it. I would let it go for the time being till it effects you more down the road. Maybe it was just being around the ow that made him drink more.