Hi ya Lyndsey,
It is very possible for someone who is depressed to continue working and cover up their depression to a point where most people wouldn't know or suspect it. I did for many years. I worked for a private mental health agency as a supervisor of two facilities for a little over 10 years. I had a wide range of clients with severe mental health problems like schizophrenia, borderline personality disorders, bipolar. I had several close friends that I worked with that knew I had depression also my supervisors did too, it never was an issue. I was really good at what I did so there was never any problems.
There are really two groups you can classify people with depression in high functioning. Those that get up everyday go to work, pay their bills, do what has to be done to survive. You can see the signs of depression if you really look for it but the person can easily mask it.
Then you have the low functioning depressive (I dont mean this to offend anyone). These are the ones who basically cant get out of bed to go to work, dont shower and take care of themselves, dont pick up around their house or pay any bills. They are totally engrossed in their depression.
If you think he may suicidal then depending on where you live...there isnt much you can do to help him unless he actually does hurt himself in someway. I can post some links on suicide & depression for you to look at from HW, perhaps if he has internet and you like you can e-mail them to him...maybe he will look at them. Just let him know your there for him and be as supportive as you can. I know its hard for someone who hasnt had depression to understand it and get a grasp on what is going on with the person you love. Hang in there...things will get better.