They are integrated GPU's within the CPU (but AMD have more of a focus on the graphics element).
With GPU upgrades, you need case space (which I'm not sure about at the moment) and power - prebuilts don't tend to have great power supplies.
The system itself isn't too bad, and I'm sure the avergae 14 year old would be happy with how it performs, but it won't be great by any stretch.
$500 is a bit on the low side for building one, which I would normally suggest (considering an OS is near $100, a decent PSU around $50, $150 for a CPU and $200 for a GPU) - all rough numbers. Still no motherboard, RAM case or storage though.
If you have spare HDDs or can push the budget a little bit (assuming you'd be comfortable putting together a build, you could do much better for him).
As an example:
PCPartPicker part list /
Price breakdown by merchant
CPU: Intel Core i5-4460 3.2GHz Quad-Core Processor ($163.88 @ OutletPC)
Motherboard: ASRock H97M PRO4 Micro ATX LGA1150 Motherboard ($62.98 @ Newegg)
Memory: G.Skill NS Series 8GB (2 x 4GB) DDR3-1600 Memory ($33.99 @ Newegg)
Video Card: PNY GeForce GTX 750 Ti 2GB Enthusiast Edition Video Card ($103.98 @ Newegg)
Case: Cooler Master N200 MicroATX Mid Tower Case ($29.99 @ Newegg)
Power Supply: EVGA 750W 80+ Bronze Certified Semi-Modular ATX Power Supply ($54.99 @ NCIX US)
Operating System: Microsoft Windows 10 Home OEM (64-bit) ($89.88 @ OutletPC)
Total: $539.69
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2015-12-31 13:59 EST-0500
An HDD would be around another $40-$50 depending on capacity - that would serve him much better than the prebuit, but I appreciate it exceeds your budget by some margin