A picture would be handy..the ones on his profile don't show it too well.
Well this is what I do:
First I take thinning shears like these: Thinning Shears - Horse.com
And cut it slightly longer than I want it pulled..sometimes leaving it ever so slightly longer in thicker areas (usually the middle)
That part is pretty hard..because of the way it cuts and you want to kind of force it to cut across the entire mane. It's better than using scissors because it allows the hairs to be different lengths which is better for pulling. I wear riding gloves for this part to protect my fingers.
Then I pull it in the style shown in this video (though I prefer to pull by hand)
Be sure to pull mostly the hairs in the back.
I hope it helps! Good luck with your guy.