secondly, you said dad said nothing, but also that dad hasn't even called to see where son is. did you not call your ex and speak to him prior to getting your son?
last thing, why is your child trying to parent this kid? Was he babysitting? If not then you should instruct him to stay out of it, unless he is being harmed or someone else is being harmed. and to probably clarify with dad and stepmom what he should and shouldn't do.