Be honest, is it just that you are homophobic or you are ashamed of being a friend of a guy.... Be clear my friend and if you are then straightaway tell him that you cannot continue with him as a friend as this is a big problem...
You're kind of an awful friend... Sorry, but it's true..
Your friend cannot help being gay. If you liked him before then I cannot understand why you cannot like him now. You do not have to have sex with him. Being a best friend means sticking with them through thick and thin.