Advice for a closet gay?

My bud and I have been friends for years now, we are really close, sometimes closer than what I feel would be classed as a solely platonic relationship. Being the idiot I am, I fell for him because he is literally everything I want in a partner. He says he is straight but we have made out before. I'm openly gay and all my friends support and accept it, him above all.What got me stumped is how do I breach the subject with him, without him being offended or causing an argument?