Does anyone know if this particular Malibu has an electronic transmission? Some trannies don't actually have a physical shift cable, as it'* all done with a switch that is made to have the "feel" of a mechanical shift cable.
I would start by checking if this tranny has a cable, located on the driver side of the engine compartment, pretty much underneath the master cylinder. The cable should attach at the top of the transmission. If you're handy with cars, try disconnecting the cable entirely (engine off, wheels chocked, ignition on) and see if you can move the lever the cable attaches to. It should be fairly easy to click it all the way from Park to Low1, with about as much force necessary as a slightly stiff doorknob.
If you are able to shift through all the gear detents smoothly with no binding (especially shifting in and out of park) then you can eliminate internal transmission damage as the cause. Next step is to move the shifter throughout its full range - any noise or binding would point towards the shift cable, it'* basically like a larger version of a bicycle brake/shifter cable (braided cable moving within a sheath) so if this cable has frayed at any point within the sheathing, or even outside the sheathing, it will give you difficulty.
It may be necessary to open up the steering column near the shifter (if column shift) or removal of the center console (floor shifter) for further diagnosis and replacement.
If you determine the shifting mechanism is fine, and the problem lies within the transmission, then perhaps you need to instruct your son not to let the car "rock back" when he parks it - especially on hills - as this places stress on the parking pawl inside the transmission and can cause difficulty shifting in and out of Park. I have to constantly lecture my mother on this one, and she still doesn't get it.
OK, there'* my $0.02 - take it or leave it.
Hope this helps you out!