This may be a stupid idea, but would it be possible for you to give him a list of things that need doing each day, so that he has a purpose and is contributing? Or would that just make the situation worse?
Is there a local mental health charity he would consider contacting? MIND sometimes have local centres where people can go and get support/ take part in activities. Or they run befriending schemes. Would he be interested in anything like that?
I used to do a list for exP. I'd put simple things that would be helpful on it plus other things like:
Have a cup of tea Text me Meet me after work Smile I love you
I think the best thing is to be supportive, try not to get too frustrated, remind him it's just a phase. It may be a bad long one but one you can work through together. And maybe try to subtly remind him if the things he's good at (subtly because it's horrible when you feel patronised).