depressed bf

my boyfriend has depression and I know that the only thing I can do to help him is love and support him through it. however, when he's depressed his entire tone while texting changes and he's not himself at all. I completely understand- I just don't know what to do. what do I say? you know? he's really down and I don't know what I should say. should I give him space when he feels like this? should I reassure him how much I love him? I don't know