When he's in a relatively better mood, suggest doing meditation together. If done consistently, it will improve his mood and help him be more deliberate about what he focuses on. This will allow him to choose to focus on more positive things.

Find a regular time you can both do it every day even if it's only 5 minutes at first. You don't need to be good at totally blanking out your mind, just constantly bringing the focus back to your breath is enough to get the benefits.