I have a strange challenge that I'm sure my fellow Radiomuters could meet with little effort.

I have a little brother who likes heavy rock music and doesn't appear to be aware of bands outside of the mainstream.

The challenge: he lives in a Christian household and there are many things I like to listen to that I feel uncomfortable sending to him while he's still living with my parents.

So...

What can I introduce him to that's secular, but has no "offensive" lyrics?

No profanity. Nothing anti-Christian in the lyrics or the artwork.

I got him Mastodon's Blood Mountain (without the DVD) and that seemed to be okay.

Any help would be appreciated!

How about Agalloch's The Mantle . Personally I think it is a very good album, it is a metal record that also incorporates some folk and "post-rock" instrumentations.
There's one song that talks about God on their lyrics, although I don't feel it to be offensive. Anyway you can check them out first:

The mantle lyrics

Also Opeth's Blackwater Park

blackwater park lyrics

The following bands have albums without profanity BUT may have questionable content:

Death
Neurosis
Ephel Duath
Melvins
Enslaved

There's probably lots of good metal sung in languages that your brother wouldn't understand, too !

Death, huh? It's true that their later stuff doesn't have the whole "sacrificial cunt" thing going on.

Maybe Human would be cool. I always liked that one.

Thanks, I'll check those out.

Don't have any Opeth myself!

How about Stryper?

Norma's Jean's first two cds are GOOD "christian" metalcore. They identify themselves as a christian band but its not annoying and it doesnt really come through in the music.

BWP lyrics could probably be interpreted as Christian by some people :P

Here is a few good christian bands. Some have a heavy style and some of these are just good bands http://www.air1.com/Music/TopSongs.aspx