Welcome to the bb forum and thank you for sharing your story. Rest assured that you have arrived at a place of kindness and support.
Your partner is really lucky to have someone like you--kind, caring and loving.
Learning about his illness is one of the best things you can do and I recommend all of materials on the bb website. The more you learn the better able you will be to understand his pain and behaviour and respond appropriately. You don't have to have all the answers, but a warm heart and willingness to listen will help. Just be there for him.
I care for a child with a mental health condition and I know how challenging it can be. The best advice I can give you is to take good care of yourself, too. It's really important to eat well, get plenty of rest and take time to do things that you enjoy. The stronger you are, the better support you will be able to provide.
I wish you and your partner nothing but the best. Depression is very common and with the right treatment most people recover.
Kind thoughts to you