If you're going to be sober, be doing something at the time like watching tv and chatting about relationships (dropping a bomb usually doesn't go well if you can help it). That way there's something to go back to if there's a bit of awkwardness between you guys.
If you're gonna be drunk, wait until you're both at the stage of talking about relationships and confess.
It's always the best policy to do it when you're both alone but any way you do it, you don't know his reaction but if the feelings aren't reciprocated the friendship will endure depending on how you handle the awkwardness afterwards. I'd be like, 'dude, it's cool if you don't feel the same way, we can go on like usual and I'll be alright'. If he's really cool about it I'd add a joke in like 'and if you ever need a bit of head let me know!'