I guess what I can do is tell you what it has been like for me what my depression had held sway. Then you can make your mind up if this person is in fact genuine, which he could well be.
The first thing is there is no timetable, it just comes. There has to be a reason I suppose but it may not be obvious, though sometimes it can be linked to events in life. When I was right down I was completely separated from the whole world - including myself. Yes I know it sounds silly or weird.
I was exhausted, I felt isolated from everything like I was on the other side of a thick glass. There was nothing to look forward to and all was hopeless. I could not understand if I loved someone or even if I could love. I thought I was a failure and everyone would be better off without me. Needless to say I'm 100% better now.
I've trotted all that out, which is rather heavy, to give you an idea of what this person may be going through at the moment. When under control one can do as Bethie says and lead a pretty normal life.
It's good news he is under treatment, which is essential. I too would suggest if this person will allow it go with him to his doctor, see what you can do to help and see what happens. In many cases just being there can make a huge difference. He knows there is someone there for him. You don't have to make big suggestions or come up with solutions.
If you do decide to help him it may not always be easy, so if you have anyone to support you that would be great. Is there anyone, parent, family or other who you can talk with? Another person's views and care can make a big difference.
It would be good if you let us know how you get on.