Can I stay lawfully, and will a judge rule in my favor?

If he has custody papers then I am not sure what you can do. However, my little sister is 13 and when my parents divorced recently she decided to stay with my dad even though the court psychologist said he was mentally unsound, he had federal drug charges against him, and he is an alcoholic. All of these were facts that were given to the lawyers, who relied on them. But that is what she wanted and the lawyers said it was OK. Little kids have too much power, IMHO, but I think that you can pick quite easily if it goes to court. Particularly if you allege child abuse.