Hi and welcome to the forum
A graphics card is the obvious thing that will make most difference. A bit more informaton is needed before we can recommend something though.
- What case is the machine built inside?
- What is the brand, model and wattage of the power supply?
- What is the make and model of the monitor/tv/screen you are using?
- Are there any particular games that your son plays that he wants the extra performance for?
- What is your maximum budget for the upgrades?
- What size is the SSD you are currently using?
Lets see what we are working with and take it from there