I reckon he’ll need to throw a leg over a 29er as i doubt he’ll need a 23″ at 6’4″.
I’m 6’7″ and have several 29ers, all in different sizes. A 20″L Kona Honzo, a recently sold 21″ Inbred, a 22″ FS Salsa Big Mama and a 23″ Spesh Rockhopper (oh, and a 24″ XXL Fargo).
If anything, my 23″ may almost be a bit big even for me (and that’s with a 39″/40″ inside leg and stupid 7ft armspan).
My mate at my LBS is 6’4″/5″ and has a XL Niner and 21″ Trek Superfly and they seem to fit him well.
In my experience FS 29ers tend to feel big, especially with 120mm travel up front and besides, it’s usually the length of the cockpit that defines the fit (hence why i can just about fit on my new Honzo in the 20″ Long version).
Rode last night with a friend on his Rumblefish and he’s 6’2″/3″ on a size 19″. It’s a tad small for me but i could ride it at a push, i’d assume the 21″ would be fine.
Unfortunately, getting a test ride on an XL 29er FS is almost impossible unless you are lucky enough to find a shop that has one in stock already and i’m guessing only Evans etc will be able to help there. Although thinking about it, i had an email from Specialized recently saying that they can get any of their bikes in any size into your local Concept store for a test session so may be worth contacting them. Niner also worth a look as they have a 140mm FS but it’s not cheap.
He’s welcome to demo my Salsa if he’s anywhere near Brum.