@
Doremon
what is your budget? and how old is your son?
does he play many modern FPS-es?
your existing setup is a little too old to be used or even modified as a mid or low end gaming setup.
even if you add a GPU, its questionable how the "experience" would be as its performance would be held back by the processor.
i'd suggest the following cheapest but quite powerful enough config
Processor AMD
FX 4300/AMD
FX 6300 6-7k
Motherboard
gigabyte 78 lmt s2 3.1K
GPU
Amd HD 7750 5.3K or
hd 7770 8k or
HD 7790 for 10k
RAM
Corsair Value Select 4GB Ram 2.7K
PSU
Antec Basiq 450W PSU 2.6k
total about 21K(for hd 7750) or 26K(hd 7790)
Keep your Hard disk, dvd writer, cabinet, and display as they are.
its not worth upgrading them