I would first ask his mom what she would be willing to allow for "punishment". Ask if it is ok if you use time outs of 5-10 minutes when he is out of sorts. Be respectful to her wishes though. If she says she does not agree with time outs, inform her that if you cannot enforce rule in your home, that you have to let him go. She may change her mind and agree to time outs.
Give his mom the benefit of the doubt and try this. Time outs can be of benefit for young children, and they also give you some control in this situation.