I would also make an appointment with the social worker from your son’s hospital. Don’t talk about your son specifically. Just describe the general situation exactly the way it’s happening (in order to circumvent HIPPA don’t use his name or the doctor’s name) and ask what to do to get your son the help he needs in the long run.
Tell the social worker what needs to happen for your son to be able to live with you, that you want to be able to find out when appointments are, schedule appointments, coordinate prescriptions, and anything else you would need to be able to do if he is living with you. Ask if a power of attorney will be enough or whether you need something else. Definitely get him to sign Release of Information for you with the hospital as soon as he is ready to do so.
Responding to medication is a really good sign!