Legal Question in Credit and Debt Law in California

If your son is mentally disabled he is not competent to sign a power of attorney. You would need to apply for a conservatorship over him.

Technically an incompetent person cannot sign a power of attorney. However, if he understands that he is giving you permission to obtain confidential information from the bank then an authorization from him to do that is all you need. Otherwise, if he does not understand what he is signing or if he understands but will not sign it then there is nothing you can do except start an expensive court case to have you appointed as his conservator. If he has no money or property that can be taken from the creditors then it might not make any difference whether or not he pays the bills.