A few possible ideas:
Rush (a personal favorite; if he's a Floyd fan and likes songs that tell a story, they might become a favorite; then again, it's not really as symphonic as the other bands you mentioned and he might not become a fan of Geddy Lee's voice)
Styx
Genesis (early stuff when Peter Gabriel was with them; Lamb Lies Down on Broadway or Foxtrot are two albums he could check out to see if he likes them)
Wilco (this is just a shot in the dark)
Radiohead
Flaming Lips
One thing that you might wanna have your son try is going to
http://www.allmusic.com, kinda the IMDB.com (movie database site) for music. You can look up an artist and, along with the Bio, album listings, etc., it also lists similar artists, bands that influenced the artist, and bands that have been influenced by the artist. He could, for instance, look up Kansas and see that Yes (bet he'd like them) was one of the artists considered to have influenced them. He can then listen to 30 second or so soundbites for every song from every album (provided that you go through the quick free sign-up steps to get a login; it's much better than listening to soundclips on Amazon since Amazon only plays clips from the first 4 or so songs on an album and doesn't even have sound clips for many). That way, he might be able to figure out on his own who he likes and will be able to discover a lot of great music on his own.