I’ve suffered with depression previously.
My suggestion would be to take him out in rides, if he is feeling that down he won’t want to go out but almost making him might work. Once he is out he should enjoy himself.
The main thing that helped me is counselling, I felt I had no one to support me when my dad took his life and the whole jump up to being the man of the family was too much. After a couple of weeks counselling I came to realise that I was putting all the pressure on me and not everyone else. It totally calmed me down.
Counselling and doing at least one fun thing a day helped me massively