How would I go about getting custody?

Okay my nephew is only 5 years old and his mother was over in Iraq most of his life. Well here recently he had a mental break down from him dad and stepmother and used a toy gun on his brother just to get back at his dad and step mom for downing his mother. Well now he's in a children's psycho ward and all he wants is his mom but his mom is getting deployed to iraq again and she don't want to hear nothing about her son. Well there's no way he can go back and live with his dad and step mom and half brother. DHS won't allow it I am married to his uncle but he needs to be around people who love him and doesn't yell, cuss, scream and sometimes hit him. I live in Oklahoma how would I go about getting custody of my nephew. What papers would I have to file and what would I have to go through? Can Anyone help me please?