Not a guru, but in my experience w/it not having power at either end of the rpm range, it likely isn't the header. An open exhaust system usually sacrifices low end torque for higher end power and higher rpm levels. If you have an oem exhaust manifold you might have him swap it and see if there is any change.
Other info that might be helpful;
Is the truck new to him? Is he used to driving a 22r? Did it have power before he pulled the head? Is he running a 5-spd or 4-spd?
