I would be cautious about assuming any 21″ frame is okay. I assumed I’d be okay on 21″ frames for years. I’m 6’7″ with a dramatically riduculous inside leg (37.1″). Riding bikes that were too small for me eventually knackered my knee, requiring an operation to remove quite a lot of meniscus. It’s never been quite right since. It’s certainly down to my particular “midget on stilts” proportions I realise, but he might be the same.
Trek and Specialized certainly do some honking great bikes. I had a 23″ Trek 4900 once. Massive but the right size for me. Just seen a 63cm Trek road bike on ebay – they certainly go big.
Also, Surly’s 22.5″ Karate Monkey fits me fine, and 29″ wheels look much more normal on a huge bike like that. Surly do a whole load of their bikes in big sizes:
26″ wheels look a bit sillier: