missydalise :
iBUYPOWER - Desktop - Intel Core i7 - 16GB Memory - NVIDIA GeForce GTX 1080 - 240GB Solid State Drive + 2TB Hard Drive -
With that chip and graphic card, and given that your son plays game at 4k ultra setting, I definitely do not think your son will be able to reach higher than 60 fps.
Because of that, you don't have to worry about the mismatch between the pc's ability to generate high fps and the monitor's refresh rate.
I think your current monitor pick is fine.
Now if your son plays game at 1080p, then certainly that graphic card will output a lot more frames per second than at 4k game setting. If that is the case, then I don't see the need of getting a 4k monitor anyway.
In retrospect, with your son's cpu and gpu...