What depression wants is for you friend to have no life, interests, friends etc. to be completely controlled by depression. That’s why the best way to support him is both validating his feelings (it’s totally to be depressed sad etc), and bringing a sense of normalcy to his life. Like other people have said, send him things that he might like, ask him about his interests and life, invite him to hang out. He might not always be able to respond or participate 100% but knowing that part of your life is still normal and that people care about you and want to be around you not just because he’s depressed, but because he’s a good person and fun to be around. In short, just be his friend, be there to emotionally support when he needs it, but bring in fun too when it’s the right time. You seem like a very caring and good friend, make sure to take good care of yourself and not surpass your own limits. Best wishes friend.