How does a father get custody of his son?

You're going to have to seek serious legal advice and not just some random internet opinions...but I can tell you from watching my stepfather and stepbrother get custody of their sons, it can be done but it's very difficult. To file on the grounds that she is unfit is incredibly difficult, she has to be really, really awful and it has to be documented. A lawyer can help you find possibly easier ways to gain custody.