Depression can be a result of lack

of control.  So start by giving him more control over his own life.  Ask what he wants to do, not what his parents want him to do.  He is an adult now.  There is nothing you can do but show your love and ask what he wants you to do in order to help.  You can help by showing that you have confidence in him.  Pesterring his school and his doctor only demonstrate your lack of confidence in his ability to take care of himself.  To answer your question in short: he is the only one who is responsible, because the alternative (i.e., the state or an institution) is not something you want.