We teach kids to learn to make decisions. Part of making decisions is understanding the possible consequences of those choices. What would be the negative consequences of buying a new iPad mini vs. a used iPad 2?
With a new mini, he would get a new device with a full warranty, newer guts and better screen, better camera and more features, like Siri. It's also a lighter device.
With his dad's iPad 2, he'd get a bigger but fuzzier screen, a used device with limited or no warranty, a lesser camera and no features like Siri. It's also a heavier device.
This question seems like less about encouraging a child to make smart choices (and exercise preferences) than a parent who wouldn't mind having an iPad mini for himself.