Most rock music tends to be about terrible teen-angst style love stories or rebellion in some form, and he will eventually get it. No point holding back, I'd say let him have access to whatever and learn things himself. Knee-jerk censorship is hardly useful, and in reality, wouldn't everyone rather their kid were the cool one everyone liked because they knew all about sex which is the first big thing kid really start to whisper about behind their parent's backs? :p
I'll second (third?) Rush, and Jethro Tull (they do have some mature subject matter*, but little to no profanity, and a lot of it will go over kids heads unless they're fairly knowledgeable and paying attention.
Some of Black Sabbath's early stuff is actually fairly child-safe (if your child is over 8), particularly Paranoid, and [i]Master of Reality[i]. Pink Floyd has an s-bomb here or there, but otherwise everything by Pink Floyd up to, at least, Wish You Were Here, and then everything from Momentary Lapse of Reason on should be fine.
If you count Electric Light Orchestra as Prog Rock, everything they did should be okay (though the opening for Fire On High creeped the crap out of me until I was 14.
*Aqualung is about a pedophile homeless man, Cross-Eyed Mary is about an underage schoolgirl who prostitutes herself. Oh, and there's all the songs about objectivism in Rush albums - and then there's Passage to Bangkok. :D
Dream theater is a somewhat child safe band, but keep away from 6DoIT "the test that stumped them all" and if your child is smart and can pick up on the meanings of songs than stay away from Systematic Chaos as it has a somewhat violent storyline to the songs, and some are about AA. wow run on sentence!
Almost all of the Hogarth Marillion is kid friendly. Fish is, too, as long as you don't dig too deep into the imagery. For example, as long as you don't tell your kids that a Magdalene is a fancy word for a whore, you'll be fine.
Maybe some 80s glam metal, like Van Halen or the Scorpions? Maybe some arena rock like Queen or AC/DC?