You don't give any dates for the claim but from what you have posted it sounds as your son is still in the 13 week assessment phase of ESA, this being the case the full rates are not payable until this period has elapsed.
If this is not the case then you should contact the DWP as I cannot at this time see any other reason for him continuing on the Assessment rate.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems