Straight guy with a gay best friend.

Alright guys, for a little bit of background, let me tell you my story. I grew up in a very small, very Christian, very conservative community. My grandfather was the pastor of the local church, and since he hadn't had any sons of his own, and I was his first grandson, everyone was looking at me to take the reigns after he kicked the bucket or retired. Again, it was a very small town. Everyone knew everyone. My 9th grade year, my current best friend moved from New York down to the small, rural Georgia town that I lived in and immediately began being bullied. Mostly because of his "Yankee" accent, but it only got worse when everyone found out he was gay, and an atheist. That was absolutely unheard of in my little town. I regretfully admit that I was one of the bullies. One day, mostly everyone, including my friends at the time, went on a field trip. I failed to get permission, and so did he. We were two of the very few people there. I decided to sit with him at lunch that day, and we made small talk. Him being gay never really bothered me. I bullied him due to peer pressure and I told him that and apologized to him, and that day we became friends. Our friendship grew, until we were seen everywhere together. A straight, conservative, Christian, southern boy with an atheist, gay, liberal, Yankee. We were complete opposites, but neither of us cared. The town, and my family began to hate me. I had to stop playing football because of the criticism I was receiving. When we graduated, he had trouble finding work in the town because everyone was such a strict, conservative/Christian. He had always been there for me, and I always stood up for him. He had no money to move out on his own, so I went with him and we left town and got a small apartment somewhere else. We both found work and we were living pretty happily. I always supported his gay tendencies and I never really cared. But one day, recently, he told me he loved me. I don't know how to respond to that. Communication of any sort has become awkward because I just don't have the same feelings about other men. I don't want to lose my best friend though. What do I do without seeming heartless? EDIT: he meant it in a non-platonic way. We've always expressed that we love each other as friends/bros, but this time was extremely different.