I thinks the first step has already been taken. He knows that he has got a problem and you want to work out the problem. So battle is half won. The next step is to consult a behavioral therapist. One of the goals of therapy is to try to promote self-confidence in these individuals by helping them realize how they are impeding their own success. The cornerstone for effecting interpersonal changes is individual psychotherapy that helps patients understand the sources of their interpersonal problems. A therapist must repeatedly point out the undesirable consequences of the patient's thought and behavior patterns and must sometimes set limits on the patient's behavior.
Here is a resource to, approaches to handling passive aggressive behavior;
Another resource to explain relationship problems in passive-aggressive behavior in married couples;
You can see that prognosis is good as mentioned in this resource;
I appreciate that despite knowing that your partner has a problem, you are trying to sort it out and not running away from the relationship. I respect your love, courage and sentiments. All of them will help you to have a smooth and great relationship. Still give yourself and your friend some time, before taking a concrete decision. Your psychiatrist specialist may suggest some drug therapy also.
Your follow up questions are welcome.