Just be there whenever he does reach out to you. It may be months, maybe even years before he does decide to do so.

Invite him out to things (not possible now), make an attempt no matter how many times he will turn it down. This may go on for a while so be patient. It will be tiring, will be annoying for you as it will feel like you're making an effort but he isn't responding but if you can do it for him, it will mean the world to him whenever he does get / feel better.

Your position is not easy to be in. I have been that depressed friend to the point where no one could deal with it and left. I do not blame them one bit and I wish I could reach out to them without any feelings of shame but I can't. I made it difficult for them to understand because I barely understood what I went through at the time. If your friend is anything like me then nothing would be better than for him to know that no matter what if he wants to reach out to you, you will be available.