@Joshy phil,
Also speaking from the experience of depression, I can offer a few suggestions for dealing with depression:
1) I cannot encourage you more to go see a psychotherapist, to get a diagnosis as to what they believe the cause is. This is central to how to deal with depression, understanding what it is. Depression, itself, is a number of different, but related, conditions. Psychotic depression would require drugs, or else the risk of self-harm becomes more and more eminent. Bi-polar (manic-depressive) disorder is typically also treated with drugs. Major depressive episodes can also be treated in the short term with drugs, to lower risks of self-harm as well. Otherwise other methods can often help, for atypical depression or seasonal affective depression as examples. But knowing what you are dealing with is the largest part, 'depression' itself is to broad to try and help with.
2) If there is a risk of self-harm, such as from psychotic depression, then drugs are necessary, regardless of personal opinion about them. There is a situation where psychiatrists push drugs without proper evaluation (which is why I suggest you go to a liscenced psychotherapist) but those drugs can save lives in cases. If it is serious or chronic, drugs may be the only effective solution and the responsable recourse.
3) Therapy can be useful, but this depends on the person. I didn't respond to therapy: openly discussing my issues raised more problems rather than solutions, to which even a trained professional was useless. It never hurts to try this "expensive" and "ineffective" solution on the chance that it will offer results.
4) As much as you want to help, interference is often not the best policy. There is a fine line between caring and prodding, one you must unfortunatly tread. The goal is to be responsive, but NEVER pushy. Rather than being someone eminently concerned, you need to be someone always available. Given the right conditions your brother should want to talk, he just needs to know that he will be heard rather than cured.
5) As much as school is necessary, depression does warrant time away from it. If school does appear to be a large concern to your brother then please have him taken out for a while if he is congenial to the idea. The last thing anyone needs is more teenagers around them when they are depressed, it is enough to send anyone further into depression rather than away from it. The damage of missing a couple of classes is considerably less than some of the damage peers can do. Credits can be re-done, trauma often cannot.
6) For mood swings, there are two things you can do: compensate and (secretly) interfere. As afternoon-special as this may sound, a pint of Ben and Jerry's goes a long way, even to someone clinically depressed. The little things get to someone in a depressive swing, so make sure they are removed and/or compensated for. Also, mood swings are when you have to worry about your brother hurting himself. Even if he hasn't before, be wary of him hoarding pills or things to cut himself with. Suicide, or suicidal behaviour, is often not an instantaneous thing but is often something prepared for.
Depression is not something to fool around with. As much flack as they get, psychologists and therapists are trained to deal with this sort of thing and their guidance should be consulted if only to have another perspective on the matter. That said, being insistent pushed people further into themselves, so tread carefully. I truly hope you are able to make progress with your brother: some of us know how bad depression can get and how fast it gets there. Just having you around will help him.
(Even in the coup chickens can be lonely. Surrounded by other poultry, anyone can feel alone. There is a difference between being crowded and being social, one not so easy to identify.)