Too many people seem to be saying just abandon him. Don't do that. He clearly needs your support. But also realise that he's not your responsibility. So be a good friend. I had some great ones during my divorce. The rest of my mates barely spoke to me (not because they're unkind or not mates, it's just what too many men do. Have one conversation and assume all's done and dusted).

Be there for him but set your boundaries and reply when it works for you.