Gertie
I think you are referring to the Severe Disability Premium. See
ESA Premiums If eligible, the premium is paid on top of your ESA, so you should contact the DWP office that deals with your ESA to find out what is going on.
That said, if your son a non-dependant and living with you you may not qualify, so should speak to your local CAB or Welfare Rights office for further advice.
Gordon