Though I agree that other's words or actions definitely impact, positive or negative, I think supporters need to be 'forgiven' to fail- to not be able to stop it or to not feel badly if they weren't available. I've heard SI described as an emotional cancer; I do believe in many cases there is much science or medicine does not understand (not just biochemically, but rather just like they're finding addictions related to malfunctioning of the reward centre of the brain, for example). I think sometimes it's just too much trauma. Maybe even related more to anxiety than depression- alleviating worry, removal of violence, fear of more loss, or the future, or harm, or betrayal. Etc. There's not much you can do to relieve intolerable (or what may feel like to him 'inhumane' ) suffering in another, except as others have said (& as with cancer) try to be supportive, be kind, be open, be honest, try not to judge, offer connection, tell him he's loved, be available if or when you can, try to include him.
Take care of yourself. :hug: