My Boyfriend has Chronic Depression

So I am dating this boy, and as I got to know him better, he told me that he was diagnosed a few years ago with chronic depression. He tells me that he's sad all the time and that he has trouble having a positive outlook on life. He also said that he's tried therapy for a a period of time and that it didn't work. I know I am unable to be the one who can cure him of his depression, but when he tells me these things, I honestly don't know what to say to him to help him to understand that I am here for him. I waas wondering if anyone here would be able to help me, or even help him for that matter. Thank you