Is the pickup going to be an offroad toy, show car, daily driver? How high on the priority list is the stereo?
My recommendation is a headunit that plays bluetooth and usb and aftermarket coaxial speakers in factory locations. Maybe throw in some speaker baffles or sound deadener. I prefer Alpine and Pioneer head units, but any of the big names will work. I wouldn't bother with subs or amps right away.
A lot of the biggest names are still prominent: Rockford Fosgate, Alpine, JL Audio. It all depends on your budget and goals. Assuming he is a young driver getting his first vehicle, I'd say let him get some time behind the wheel before you go too crazy with your build/restoration.