Fortnite demands a good to better GPU. I don't know the rest of your PC. Some need a better PSU, some may need better case cooling but in short, you read this list and go as high as the budget allows.
An Xbox may be cheaper. https://www.xbox.com/en-US/games/fortnite
How can myy son run fortnite on x84 processor.. or what do I need to buy to make it compatible?