Ssi is a needs based system so if his means exceed allowances, he would be disqualified. Since he is a minor, your assets and income are involved in the calculation.
Unless ssi determines his disability is a permanent situstion he is subject to regular requalification.
Then, as representative payee you are obligated to maintain accurate records for
The use of the ssi payments. They are always subject to review by the ssa. How much he has in his bank accounts is also regulated. So, they aren't doing anything out of the ordinary here.