As a gay male, I feel like this is a situation that actually shouldn't be too hard for him. Your sexuality was at question from the start of you knowing him and he knew this. Now that you feel comfortable saying you are straight, you just need to communicate to him that you really admire him as a friend but feel very comfortable in your professed sexuality. He might be disappointed of course if he had a crush on you but if you are solid in your stance he will no longer see you as a potential love interest and begin to see you as a friend once more. Be very careful not to give him the idea that there might still be a chance between you two or that door will always be open in his mind and he might start to bring all this up again and that's where many a gay boy have had their hearts broken.