can the graphics card be upgraded?

The laptop you purchased is definitely more powerful than the old dell inspiron 15r 5520. If your son was able to play Minecraft on your old laptop then he will definitely be able to play it on the newer laptop. The newer generation CPU is a bit more powerful even though it and the older i5-3210m CPU both have the same clockspeed. However, the difference in performance between the old Intel HD 4000 and the current Intel HD 520 is much greater. It depends on the specific game, but overall the Intel HD 520 is probably around 45% - 50% more powerful than the Intel HD 4000. Most games are limited by the graphic core.

If you want something that is more powerful than the Intel HD 520 graphics core and under $500, then you can by the following refurbished Acer laptop with a "Core i5" CPU and a nVidia 940m GPU. There is no model number provided, but since it has a nVidia 940m GPU, it should no older than 4th generation Intel CPU, but I suspect it is actually a 5th generation CPU.

https/www.amazon.com/Acer-Dual-core-Win10Home-Certified-Refurbished/dp/B01BUGB81C/ref=sr_1_9?ie=UTF8&qid=1476814025&sr=8-9&keywords=acer+940m

Regardless if the CPU is 4th or 5th generation, this laptop will definitely provide better performance in Minecraft than the one you recently purchased due to the more powerful nVidia 940m.