Welcome, Markus.
First of all, I know that your situation is difficult. There is a real danger that you too can become depressed or feel like a failure. Please fight this idea, and never give in to a possible urge for isolation. He is the one with the diagnosis.
In our Outside Resources on the left, there is a link for Families. There are other links there too for him.
If you live in the USA, there are Family Resources on the NIMH site. In Canada, look up CMHA.
There are more and more resources for family, but I'd bet there is still a great need for more. I was introduced to a fledgling network of "Families and Friends of the Depressed" a few years ago. They were forming a network and through national connections were developing a video and book library, were forming local support groups, and providing advocacy, legal information etc. It was a godsend and I urge you to search until you locate something like this in your area if possible, and if not, online.
I was a friend who "accompanied" someone with depression for a long time, (he's much better now)sometimes being the confidant, the advocate, the protector, companion, entertainment organizer, the encourager, etc. I found it really offensive that when I first met him his best friend was the TV. In our exchanges, I found it helpful not to give unwanted advice, and I avoided criticism, judgment and arguments. In my situation, I spent a half-hour per day just listening on the phone, or sharing a coffee and being a positive encourager, and a calm resource he would come to trust. I asked his opinion on things and was amazed at some of his insights. I planned routine brief outings on a weekly basis that were interesting, like attenting seasonal activities. He helped me too, with grocery shopping and small tasks errands, etc., and felt he was my friend.
I was able to make non-critical observations for him to tell his doctor like slurred speech, changed gait, pallor, loss of appetite, or blurred vision, for instance. He accepted the idea of my slipping a reminder note into his jacket pocket in the hope he would remember to tell his doctor, especially in observed changes that could well be medication-related.
I hope this helps a little. All the best. Bless you, Marcus. Keep