You specifically mention American in the title, but don’t say anything about it in the OP and even mention Pink Floyd who isn’t American. But, for what it’s worth, there’s TONS of stuff that’s great and new.
In the vein of Pink Floyd, I’d recommend Porcupine Tree. Much of their earlier stuff is highly influenced by Pink Floyd. Their last 3-4 albums though have been distinctly more modern rock and metal influenced, but still very artsy and psychadelic. For their more psychadelic stuff, check out an album like Up the Downstair, Lightbulb Sun, or The Sky Moves Sideways. For their more modern rock sound, check out Recordings, In Absentia, or Fear of a Blank Planet.
In the vein of Dream Theater, they’re simply the best of the modern Progressive Metal bands. If he likes them, I would recommend a number of bands. Spock’s Beard is from the same area, but they’re a little more rock oriented and just have some bizarre melodies and jams… some of it’s a little too goofy to be taken seriously, but it’s generally pretty solid and a band that a Dream Theater fan should like. I’d highly recommend Octane and Snow.
In general, most Dream Theater fans will probably like some or all of these bands as well: Symphony X (The Odyssey, Divine Wings of Tragedy), Ayreon (Human Equation, Universal Migrator, Into the Electric Castle), and Pain of Salvation (The Perfect Element, One Hour by the Concrete Lake, Remedy Lane). Depending on which of those he’d like, I can make more specific recommendations.
To anyone who is a fan of Metallica, it’s really hard to say because metal has become SO incredibly diverse these days that I really don’t know where to go. Would he like more of the modern thrash sound? Would he like Power Metal or Death metal? Either way, I can 100% guarantee that any fan of old school Metallica will like Iced Earth. He should check out The Dark Saga (a concept album about the comic book character Spawn), Burnt Offerings, or Something Wicked This Way Comes.
I could make a ton more recommendations, but it really depends on what he’d like. If you just want a list of some kick-ass bands, check out my post in the metal fans thread.