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Hi there! I hope to offer some suggestions here; this is definitely a situation that will require some very creative and intentional thought and planning, but there is hope!
At the age of eighteen, he is still somewhat reliant on you and clearly needs some help right now. It sounds as if you have tried a variety of discussions with him, and this hasn't worked. Now, it may be time for some "tough love".
If there are some stronger family members whom he looks up to, I suggest a family "intervention" meeting. This is done without his knowledge, and takes place in a somewhat dramatic fashion with everyone converging on him at once. Every takes some time to express their concern in the group, and to tell him you will now stop taking "no" for an answer. You have decided that it is your responsibility to get him the help he clearly needs, and you are now taking him to either a Dr. (appt. set up prior) or to the emergency room, if you feel that there is suicidal intent.
If he has expressed a suicidal ideation, or a plan to hurt himself, rescue should be called and him taken to the nearest emergency room.
You can simply explain to him that his body chemistry is probably out of balance, which is causing this depression. Explain that the symptoms that he is demonstrating are absolutely indicative of a depression, and that his body needs help to get back in balance. Make it clear that this isn't his fault, but that when people are sick, they sometimes can't help themselves and must rely on and trust others to help them. And you are all now going to do so.
Don't offer him a choice any longer about getting this help; just make it happen. He needs you to be strong and offer confident leadership right now; and unless you take some action, things could possibly get more serious.
I hope you found this answer somewhat helpful. I wish you all the strength and courage you may need to take control of this situation, and help him. Please feel free to reply with any further information or questions, and I will be happy to reply as soon as I can! Hang in there--- you will get through this!