It depends, there is two options, if you feel something for him or if you don't.
If you feel nothing for him but a friendship, the best thing would be to let him know that you care for him as a friend and don't want to lie or hide anything from him and that you need his support and make sure you let him know that even when you are gay you will respect him, that he will never expect to have a wondering eye when he is changing in front of you, of if he looks good with his need shirt and you noticed it. Try to make it as little uncomfortable as possible for him. You need to understand that if he sees you as his best friend, he would't care who you sleep with and he will always support you, so I think you should tell him, not with drinks, not in a private place, don't make him feel this is about him, do it while walking on the street, at school, at work, maybe in front of someone else that may know about you. Or if you want to do it alone, do it, but make sure it doesn't look like you are trying to seduce him, and allow him to ask questions.
If you have feelings for him, forget about telling him and step back a little bit, otherwise you will lose him as a friend an as a potential lover, boyfriend, etc. However, if you think he might be, you can try to ask him first if he would do something gay or something like that, or if he has tried something in the past or whatever, first you need to check if there is a chance for him to do it, and then if it would be with you, after that you can tell him about how you feel. But if you know that he will never do anything because he is not in the same place as you are, you need to step back and keep a distance, even when it will hurts like hell, because there is no other option but for you to get over him and move on, you don't want to stay there waiting for a miracle to happen. Don't lose your time.
For whatever scenario you will go through I wish you the best of lucks, and as a person that was in love with his straight best friend for 2 years, I'm telling you all this from the bottom of my hart and the heart of my bottom.
Cheer up!!! And whoever loves you will remain by your side, Don't forget about that.