My first question is: Are you witnessing any of this for yourself? If not, who are you getting your information on mom's behavior from? Given your reference to her isolating all of you, I suspect that your source of info is the teenager himself. As such, I am instantly suspicious as I have worked with teens for far too long to take such claims at face value and most often find that the road is a two way street with blame enough to go around.

Now, if he is being physically abused or not provided for (i.e. neglected) the proper course of action would be to report this to the proper authorities. Mom being controlling or overly strict is not grounds for removal from the home or termination of parental rights. If mom agrees to a different placement for her son for a time, that can be acceptable. Absent that, it will take an action of the court to remove her parental rights - at least temporarily - for him to live with anyone else. And, then, it might not be who HE or anyone else in the family chooses. If she does not agree, then it appears that KS does not permit for his emancipation absent mom losing parental rights.

If you feel that he is being abused in some way, notify your county's child services agency of what you know or suspect and let them look into it. That's about all that you can legally do.