What can I say to him?...?

I have been best friends with this girl since I was 5, for the sake of this question let's just call her Brit. Her brother (lets call him Dan) has always been very close to me, he is one year younger than us and since I was young, the three of us would hang out and sleep in their basement. Brit would always fall asleep first and me and Dan would stay up all night talking and laughing. He has been the only person who has ever really understood me fully. A few years ago I had a party at my house and he kissed me and one of his friends said that he was in love with me, but I was too vain to tell him that I liked him too, but the truth is that I do really love him. Now I am not sure he is interested in me anymore, but I can't stop thinking about him. I think I am going to tell him how I feel. I just don't know what to say. Does anyone have any advice??? Guys especially-- I would love to get your opinion on this. thanks so much in advance.