Just be honest with your friend and be ready for him to respond. He might be surprised, he might be relieved, or he might be uncomfortable. But you owe it to both of you to talk about what's on your mind and work it out together. Your conversation might be great. It might be awkward.
Real friends can talk about everything, even when it's difficult.
I have always told myself I would never date a non-Christian, but lately I've been wondering if it's all right. There's a guy at school who is a wonderful person and honestly one of the sweetest guys I've ever met. He has high standards like I do, and accepts what I believe, but I don't think he's a Christian. I've been praying about this a lot and have asked Christian friends for advice, but I'm still not sure what's right. But the Bible isn't very clear about dating.