How do I let him down easy? (Details inside).

I have recently acquired a phenomenal boyfriend; he makes me happy and I love to spend every moment I can with him. However, one of my best friends is a guy. We were once “friends with benefits” but we decided to let it go. Last time I snagged a new beau, he cut off all contact with me and didn’t talk to me until that relationship was over. Now, I am stuck hiding my relationship from him just to keep him as a friend. I really don’t like living this way. I wish he would just be happy for me, but he believes he can’t be my friend unless I am single.

How can I let him down easy and still be friends? I don’t want to break up with my boyfriend just to keep a lasting friendship.