My son cleared the code after replacement, and it came back as a pending code.

Steve

 

Some codes need to happen in multiple drive cycles to go current..
If it is not current don't try to over think. IIRC; the P03xx codes need to occur 3 times in a drive cycle to light the CEL. It has to be bad enough to possibly damage the cat.
P0300 is NOT "multiple" or "random" misfire, it IS "misfire detected" but not specified. Misfires are diagnosed by variations in the crank sensor. Has the crank sensor or the ECM been replaced without relearning the crank sensor? You can replace the sensor with no code, you can replace the ECM with a preprogrammed ECM without a code; the only code it has is "not programmed" if it is a virgin ECM. Most tunes delete the "not programmed" test.

And, one more time don't clear codes. They clear themselves when fixed. Only read codes when the CEL is on, don't worry, have an adult beverage instead.