Do I need a special needs trust for my autistic son, if he is only the POD from my cash bank accounts?

That depends on just how competent, or not, your son is;  if he's profoundly autistic, the trust might be a very good idea.

However, you can also use your will, to name a custodian for your son, if he's not competent, after your death.  That custodian would also take custody of his money (including the $30k from your accounts), with a strict legal duty to use the money only for your son's benefit.  In fact, where substantial wealth is involved, sometimes there are two separate custodians named, one for the child's person and another for the money.