There are people in this world who live alone for many years and grow accustomed to it. Kahlil Gibran wrote, "Love knows not tis own depth until the hour of separation. As long as he's not hurting himself or pursuing some sort of ultimately self-destructive goal then just support him best you can and let him work these things out for himself.
We believe hope is the bridge between loss and recovery. Welcome to loneliness when he has over the original and fellowship. I don't think it's necessarily right to say that he can't be in a relationship now. This article is as powerful in as it was when first published 13 years ago. In my family, they don't even wait until the spouse is dead to start dating, or shacking up. Friends were shocked, but then they weren't the one's who'd just lost a long-time companion.