I'd say the most important thing for you to do is stay in contact with your friend. You will probably have to do most of the planning and initiate most of the contact. It won't mean that he doesn't need your friendship, just that depression makes people passive. Your friend needs contact with people, it is really important.
If you can get him outside the house and doing something stimulating, that is great. But don't push too hard - him feeling pressured is not what you want. As for being positive, that is great, but again, don't go so overboard that he feels pressured to be the same way.
Just be a friend. You can't make the depression go away, but you can be a positive presence in his life, and he needs that just as much as the Zoloft.