I think that first and foremost you ought to encourage honesty from your child. If he trusts you enough to talk to you about it, then he will trust your opinion on underage drinking more. Start with that!

Your 19 year old is probably nervous that you'll be angry with him for drinking. Make sure he knows that you just want him to be safe. You're not going to judge him for drinking, even if you don't like it. Let him know that you're aware he's drinking, and that you wish he had been honest with you. If he knows that you're not going to try and punish him for drinking as long as it's not out of control, then he may be more inclined to be honest with you in the future. 

Try and emphasize the importance of honesty to your relationship. Show him that what you care about is that he's safe, not that he's following every rule to a t. Let him know that you understand that teens drink alcohol in college and that you want him to be smart about it, but that you're still supportive. That's a tough situation, good luck! 

I would definitely make him think he can be honest with you without getting into trouble.  You need to reassure him that it's not a trap, and tell him that you know, but in a soothing way.  Laugh about it, "you meant to tell me you went out with someone that drunk, and didn't have one??"  Make the space seem safe and you'll get so much more out of him 

Also, if you had experiences in college with this you can give a modified version of it. I don't think children need to know about their parent's past lives necessarily but I have told my children a little bit about my college days. That it is normal to try things when you are a young adult but that you need him to be safe with his choices - especially about driving home with someone who has been drinking. I find this a tough age to parent. Good luck!