my ex kissed me...

I want to be friends with my ex. He was my first love and I will always love him. My current boyfriend is perfectly fine with me hanging out with him, he knows my feelings towards him, but today... he kissed me. I pulled back as soon as I could react, but I don't think I could tell him because he didn't try again or anything, I would really like him and I to stay friends, but if I tell my current boyfriend about it he might say we can't hang out. What should I do?