My son has started calling my brother dad?

I am a single mother my son is 16 my ex husband died a year ago in a n car accident i also have a 9 year old daughter my son has been depressed after his father died after 2 suicide attempts he is in therapy very close to my brother has become more like a father figure to him he takes him fishing and golf they do everything together like a father and son would do. A few months ago my son asked me if he can spend the night at dads house. I didn't knew what he was talking about i asked him if he was ok he was fine then my brother called me and told me the whole story about him calling my brother dad i feel really weird about my son calling my brother dad what should i do about this?