Mac Pros are different. They are partitioned off internally into separate "zones" and each "zone" has independently controlled fans. PCs are usually just one giant open space, so heat from different components is free to spread.
But none of the temps you give are outside of what is considered normal. They're actually into the good territory. They're also inline with your system.
88F is ~31C and 121F is ~49C.
There's nothing wrong with your son's system, it's running at near identical temps to yours. The fan is working a bit harder, but that's because of the partitioning in the Mac Pro case. If you removed those partitions, you'd see your fan speeds go up. Plus Apple tends to do some funny things with fan speeds. They intentionally keep them low to cut down on noise. I also think Apple tends to use larger fans, so more airflow at lower RPMs. Believe me though, those Mac Pro fans can get spinning at a good clip. I was working on one once, and as a diagnostic was running, I leaned into the case to trigger some in-case diagnostic LEDs. As luck would have it, that's when the diagnostic program started the fan test, and gave me a nice face full of dust.
While 70C is a bit on the warm side, it's still within pretty normal ranges. Once you get above 75C, then it's time to start rethinking your cooling strategy, but most CPUs and GPUs should be able to take up to around 90C.