Just do the best you can with him. I am in the same situation with my stepdaughter - age 10. She has no rules,is very spoiled at home and does what she pleases when she pleases. Her mother hates us and actually sets this child up to act disrespectfully at our home. She tells her to tell me she doesn't have to go by my rules because I'm not her mother and if she gets mad, she calls her mom from her cell phone.........yes - mom gave her a cell phone to bring to our house so they can talk when she is with us. Its a very sticky situation.
Just explain to him that his mother loves him very much and that is why she sets the rules. If she didn't love him, she would not care what he did. How soon was the divorce? Can it be that he is just adjusting? Is there a new spouse in the home? Maybe dad is telling him how awful mom is and is causing the hard feelings? Its awful when parents put their children in the middle.