Sorn do i have to tell the dvla and child benafits?

hi my stepson has lived with us for 6 weeks his mother is refusing to give his benefits up .we have filled in the forms for child tax and child benefit but this will take some time to sort out ,what can we do . now sorn and my car i have a car because i am disabled and my partner is my carer and on income support we cant at the moment afford to insure my car although i need it badly .i put my car at my mother in laws bungalow she owns her own house and has a drive the car is parked there .so do i have to sorn it please as i am disabled i get my tax free do i have to send the tax back .thank you .this is important .