I also have these bouts of depression. They come and go. What works for me is keeping busy. Just doing anything helps me. Just move. Laying in bed is the worst thing to do when that happens. This is my advice to him.
My advice to you is the following: Be there for him. That is all you can do. If you were physically close to each-other, I would suggest to just snuggle/cuddle. Human contact helps. But you are in a long-distance relationship, so that is a no go. Send him voice messages instead of chat? Let him know you like him. Let him know he is important to you. Let him know you are there for him.
Regardless, thank you for caring about him. Compassion, empathy and kindness are the best qualities anyone can have.