Depression is a serious condition.
Perhaps you could encourage him to see a psychiatrist for medication and a counselor to talk to. If he does not want to talk to a psychiatrist, a family doctor can prescribe medication for depression.
Depression causes people not to want to engage socially or do the things they used to do. Try to encourage him to stay connected with other people and to do things that he likes whatever they are- hobbies, sports, etc.
What To Do When Someone You Love Is Depressed : A Practical, Compassionate, and Helpful Guide by Mitch Golant Ph.D. and Susan K. Golant (Paperback - Mar 15, 1998)
If you reside in the US and have a valid concern that he may hurt self or another person, you can find out in your state of residence how to go about getting involuntary help for him. That only lasts 72 hours and if the person proves to be fit they release them.
If possible, perhaps the roommate may assist you at least in convincing him to go to a regular doctor for medication.