It is unlikely that you will be able to replace him with medication, which is often necessary for depressive states. But you can push him to turn to a specialist. Although, I know from my own experience that this does not always work. My dad has been severely depressed since my mom died, and he doesn't leave his room for days and doesn't even turn on the lights. But, thank God, he keeps in touch with me. So I downloaded the backgammon application here https://captain-droid.com/en/games/ and every day I come to it to play a few games. The main thing is to show the person that you are there and that you care about him. I think this will be a small step towards his recovery.