Originally Posted by MadeOfGlass

He's mean and bitter because the drinker in him wants a drink, craves one, and strongly. He's fighting the fight, and he's close to losing it, looks like, with that beer in your car, if he hasn't lost the battle already.
Now he's looking for the green light from you that he can have a drink and you won't parent him, chastise, or start a battle.
He doesn't want to admit to himself that he's about to lose the battle all on his own. He feels bad about himself because he wants to renege on that speech he gave the whole family. The drinker in him regrets ever making that speech, and wants to find a way out of that promise.

If he brings up drinking, I would ask him if he would consider attending AA. What I would do for myself is prepare myself mentally so that I wasn't surprised when he relapsed.
He regretted drinking with the DUI. Should that be enough? Not really...now he may think to himself the thing to avoid is drinking and driving together, not necessarily ceasing drinking altogether. That's the drinker talking. That's what AA is for, to squash the drinking side of his personality and bring out the side that can choose differently.

Alanon might be for you, to help you learn to cope when/if he starts drinking again.