(Updated Post):Well,I don't think you should upgrade your monitor but instead upgrade your CPU.
(at the same time I agree upgrading the monitors too but please take a look at this post).
Your CPU i7-3770
ARK INTEL
Performance
- # of Cores - 4
- # of Threads - 8
- Processor Base Frequency - 3.40 GHz
- Max Turbo Frequency - 3.90 GHz
- Cache - 8 MB SmartCache
- Bus Speed - 5 GT/s DMI
- TDP - 77 W
(4 cores can handle RTX 2080 TI? Especially that the the 4 cores Turbo boost are barley 4Ghz)
I thought of this because when I was looking at stats on how much FPS would you get in AAA titles using the Build you have.DDR3 32gb Ram is overkill but future proof so no problems in performance with that much amount of ram but, its ridiculous to me if you want to upgrade your ram consider changing to DDR4 (you don't have to but your ram speeds are multiplied with higher DDR numbers currently DDR6 is max and don't buy it DDR3 and DDR4 is enough, DDR5 and DDR6 ram is usually for more consistent performance and the ability to make the GPU reach its max running power without any problems).You don't have to change your PSU since its More then enough.Kingston SSD doesn't matter to performance but it makes you transfer data faster then normal depending on Your wifi upload speed.A TB Hardrive is good but more then enough but still
its a nice addition.
The bottlenecks are near 20% and I recommend to upgrade to a intel i7-6700k or i7-8700k or i7-9700k the over clock versions 6th gen of i7 can work too.(only if your motherboard can support it.)i7-4790k etc.
if you cant buy those I recommend you to get a AMD CPU only if your motherboard can support it.
Ryzen 7 3700x is a Ideal cpu for it or the expensive Ryzen 9 3900x or you can wait for the better one Ryzen 9 3950x,Ryzen 7 2700x etc.
If you experience bottlenecking you should change your CPU running power if it bottlenecks the GPU if the GPU bottlenecks the CPU consider overclocking the CPU to higher speeds then normal.Like I said, A 900 watt PSU is good and more then enough.You cant overclock the CPU where it requires more power then The PSU can.Intensive GPU,RAM,CPU Overclocking probably needs more then 1000 watt power supplys but im not encouraging you to upgrade your PSU though its enough for now and for the future.
Newer Motherboards may also increase the performance depending if its used for a long time or if it damages it can help transfer power faster and bring data faster.
Higher Resolutions can help keep the games running smooth but the PC may get louder(such as fans at 100 percent max usage) because the GPU needs to work harder to handle the more detail and fans increase in RPM to keep the heat out and keep minimum airflow.(VERY HIGH POWER on a component of an motherboard can damage it and likely damage its lifetime fixing cpus and gpus are hard especially when opening it up and checking the board to see if there any burns).Right now if you don't want to listen to this post and listen to the others about the ones saying use higher resolutions on your computer to smoothen the performance I agree with them but im including the upgrade on the cpu so you can get more performance out of the GPU.
Monitors are expensive especially you want that 1ms 4k 244hz LG or ASUS Monitor lol but seriously you don't need to spend that much especially for those kinds for people who want to make money off of competitive online gaming like the Fortnite World Cup.
The only time you want higher hz is the Laptop Linus Tech Tips made a review on where it can reach up too 300hz refresh rates but since Your son doesn't want to change all of his components its just a random suggestion or a reference.