[Eternal]

MDMA essentially uses up all your serotonin at once, which causes a huge crash afterwards. You're basically using all of your "happy" at once. How often is he taking it? I think you should try and encourage him to talk to a counselor at University. It should be free, and typically they only do short-term so it wouldn't have to be a big commitment for him. They also can't report him if he decides to talk about his drug use.
That's all you really can do is just be there as support and try and encourage him to pick up healthier habits. Addiction can be tough to get rid of, especially alone.