Robert R McCammon is a fantastic author and Swan Song is his post apocalypse vision that in my opinion, ranks up there with Stephen King's The Stand. I read this back in '93 and it is one book that has stuck out in my mind all of these years. Definitely worth a read, but good luck finding a hard cover edition unless you are willing to pay big bucks for it! Nonetheless, the paperback or e-reader versions are well worth your time.
Michael sent Adam's soul up to Heaven with his mother before using his body. When Michael said "Adam's not home right now," he meant it. Adam never got stuck in the Cage, that's why we never heard anything about him after "Swan Song".