With that little air, he'll probably just have to wait till you get there before doing any jetting.
I'd get a couple sizes smaller mainjets to take with, and a smaller pilot jet.
My experience has been that the pilot goes down one, mainjet down one (or two), and sometimes the needle drops a position also.
Of course, we've had two of the same bikes (XR 250's) at 8,000' and they both took different jetting (completely stock bikes, same year). This is why I say wait till you get there.
We can throw rocks at it, and get it close, but fine tuning is pretty essential up that high. Don't forget spark plugs! Even with re-jetting a few of our bikes fouled plugs last time we were up that high.
BTW, lower octane gas is better at elevations that high. You probably already knew that!
I ride an XR400 with an aftermarket exhaust and an unrestricted intake. My normal elevation is around 1000 ft. I run a 160 main and a 60 pilot for these conditions. I rode an enduro last year near Cloudcroft NM. Elevation ran around 8000 ft. and temperature was around 50F. I wound up with a 150 main, 50 pilot, and moved the needle clip up one position from stock. The clip position change really made it run right.
Trailryder42 regularly rides an XR400 at higher elevations. I believe his bike is closer to stock. You might drop him an email.
I just came back from Colorado last week, and most of us ended up running a 160 main (2 yzf 250's, my wr500). We normally ride at around 800 feet sea level in Oklahoma and all run 175's for mains. I was told to use 1% leaner for every 1000 ft difference from where you live. That tended to apply pretty close for our riding. The thing about Colorado is that there is so much elevation change, your jetting will be good sometimes and mediocre at others. With that in mind, I stayed a little rich and ran a 160 instead of a 158. None of found the need to touch the air screw or the pilot on our bikes. No fouled plugs between 8 bikes for 6 full days of riding (100 miles per day, from 6000 feet to 14000 feet).
After several trips to Co. , I've found my best jetting to be 150/50. After I dropped the needle 1 position too, it really woke it up for those altitudes. I have the "Gordon" mods on my bike. Just an addition, My gearing is 14/47.
I'll post this in lew of an e-mail. I'm running leaner main than most here 145 (if I remember right) 52 pilot and air screw 1,5 turns out.
I know dyno's are subjective but this combo works for me. Last plug pull showed a nice healthy tan color.
If you don't want to invest in jets, let me know when you'll be out and I'll set you up with a wide range so you can try a few. I have quite a collection of them. Let me know. (I know your good for getting them back to me ) ;)
I'll be in Canada 7/19-7/30 so if you don't get a quick response wait a bit
I would strongly reccomend dropping one tooth on the CS sprocket too. It will help compensate for the altitude