Hi lolau22
As your son has been awarded the middle rate care of DLA then you would be entitled to the low Disabled Chils Element £128.89 pcm.
You need to ask for a revision/supersession on the grounds that you are now in receipt of a qualifying benefit. I would add a copy of the award letter on your journal.
Gary
Nothing on this board constitutes legal advice - always consult a professional about specific problems