Don't know how is your communication with him. You don't have to worry too much into this matter. May be he has some other underlying issues that you are not concerned about. Being a foster parent, you have to intervene into the matter very efficiently and carefully. Programs like Home based intervention plan can help you changing his deviant behaviors. Like Home Intervention System which is developed by administrators of schools and programs for struggling youth and has been adapted for home use. The Home Intervention System shows you how to motivate your teen to make changes just like they do in these schools and programs. The System utilizes a simple but powerful Attitude and Behavior Modification Program

I hope this information helps you..