You stumbled because you saw him buying beer? That's strange, how did that stumble you? Did you think "ooh, he's buying beer, maybe I should buy beer too despite the fact that I feel conviction about this"? Or by "stumbled" do you mean you just didn't like seeing him buy beer? I guess I never understand what people mean when they say that they're stumbling because of what someone else is doing.
First off, do you even know that this guy is a Christian? He could just be some kid who is curious about God, or he might be just going to church because it's church and he has to. From what you said in your first post you don't seem to know him at all, my advice to you is get to know this guy and where he's at before you call him out on something he might not even be guilty of! Maybe you should go up to him and try to be his friend before anything else, maybe nobody else has come up to him and made friends with him in the whole church and he needs to see some genuine Christ-like love before he's able to give his life to Christ. This is just speculation, but from what you've said it doesn't seem too unreasonable.
Secondly, buying beer shouldn't be considered a sin at all, in fact check out what Paul says about drinking alcohol in 1 Tim 5:23: Stop drinking only water, and use a little wine because of your stomach and your frequent illnesses. Hmm... that's interesting, so it's ok to drink a little wine now and then? God created something and we're supposed to use it for good? Yep. Now Paul's not saying go get drunk, in the context of this verse he's recommending drinking a little wine to stay healthy, meaning that it does have it's purposes and shouldn't be hated because of the way some people use it to get drunk.
Of course Paul has plenty to say about not getting drunk as well, he says that the deacons of a church not to indulge in "much wine" (1 Timothy 3:8). He says that older women shouldn't be addicted to "much wine" (Titus 2:3), and he says in Ephesians 5:18 "Do not get drunk on wine, which leads to debauchery. Instead, be filled with the Spirit.".
But actually, he never says that getting drunk is a sin! He says that it leads to sin, but I'm not so sure that getting drunk is a sin. In fact, Jesus gave the people at a wedding party more wine when they were already drunk (John 2:10)! Would He intentionally cause somebody to sin? Certianly not! However, I don't recommend getting drunk at all and I don't see the appeal in it as a Christian. We shouldn't rely on anything to escape reality or need to rely on anything for comfort other than Christ. Please please please don't get drunk because I told you I don't think it's a sin, like Paul says in Ephesians 5:18, it only leads to sin, don't walk a line you don't have to. And plus, you could potentially cause a brother to stumble into a lifestyle that Christ conquered in him! Alcoholism is an addiction, and a man is a slave to whatever has mastered him (2 Peter 2:19), and since being drunk leads to sin by being an alcoholic you're enslaving yourself to the sin that Christ set you free from (Romans 6). Therefore, it's best not to get drunk at all because of all the sin it leads to and because eventually you will be enslaved to it, and it very likely could destroy your life. Plus it's really bad for you.
That being said, it's perfectly understandable why most Christians have a problem with alcohol. Read Romans 14, I really like what it has to say about this issue. You should read the whole chapter, it really says alot about the subject of personal conviction and judgment. At least read from verse 19 to the end of the chapter though. Basically, if it's going to cause a brother to stumble don't do it around them, or at all. But I'm pretty sure "stumble" means cause them to sin, I don't think you can say "he stumbled me" unless you're talking about someone buying beer and causing you to become an alcoholic. However, you're not a victim if they do "cause you to stumble", you're still the one stumbling, they're not at fault for your sin.
Whew, I have a feeling I've just redirected this thread from being about judgement to being about getting drunk. But all that I said was still on topic and needed to be said. I hope the length of this post doesn't scare people away, I said a lot of things that most Christians shy away from.