OP, presume you don’t mean factory air ride, because you mention a gauge which the OE systems do not have and you can’t “pump them up” unless the system has been hacked into.
Assuming aftermarket bags and a (relatively) light/ no additional load in the truck, your post makes zero sense.
30psi, much less 50 or 90 will Jack the back of the truck up sky high.
Zero psi should result in normal operation.
So unless the springs are totally shot ( still not the issue as 90psi and NO springs at all would Jack the truck up ), you must mean aftermarket air ride like a Kelderman system or similar? (But you analysis still doesn’t make sense.)
And shocks have virtually nothing to do with ride height. They don’t support any appreciable load.
You n the son should take a good look at the truck and do a little research. Whatever the issue, it will be obvious once you spend a few minutes understanding the rear suspension.