That is a very typical symptom of depression. I was depressed for about three years. Mine was due to a tough relationship breakup. I pushed EVERYONE out of my life including friends and family. I am no longer depressed and am reconnecting with the world now.
It is extremely taxing on everyone. He is very fortunate to have you and that you are being so supportive and seeking help for him. He may not feel comfortable speaking to anyone about his depression because possibly he will have to face what he is depressed about. In my case, I would have rather stay in my depressed state than to face the fact he was gone. When I finally did face it, I let him go. I felt so much better. It is possible your boyfirend may not know why he is depressed at all. It could be a chemical embalance. Try to encourage him to get some help. He may push you further away in the process. Try to stay strong. I'm sorry, I know it's hard. He is definately not alone in the way he feels. You could have him come here and ask questions. Maybe he needs someone that can relate to what he's going through. Therapy helps many, meds for others, sometimes both. If depression is due to a loss, time cures pain as well. But don't let it go too long. We also remain here for you.