I agree that just being there is very important.Could you drag him out for a walk in the countryside (especially nice at the moment with all the Autumn leaves!).
One of my DC has in the past gone through some very tough times emotionally, threatened to kill himself etc. At a loss for anything else to do I said well come for a walk with me now, you're not losing anything by putting it off for a couple of hours. So we went for a long walk and to be honest we both felt a lot better and calmer. There is something very beneficial about gentle exercise in a natural setting. We do it regularly now and sometimes he will come and say I've had a hard day, can we go for a walk. We both enjoy the quiet time together (we don't usually talk much, just a few sentences here and there). It's even better with a dog of course.
Worth a try? If he wouldn't naturally come, you can always say that you'd like to walk in the woods, but would feel more comfortable with someone with you.
Ultimately could you get him to join a local walking group with you (and then you gradually step back a bit)? It's hard, but after a period of grieving he needs to start building a new friendship group and move on. You can't tell him that, but you can put him in the position where circumstances make that start to happen.
One other thing to consider might be volunteering - especially around a big fundraising event, maybe for a heart charity? Something where there are lots of people involved and it has a certain momentum. Again I think you sign up for it then say, we desperately need more people for this, it's just until March (or whatever) drag him along and get him involved. Hopefully it will give him new connections and a longer term involvement in something outside himself.