I don't know why you're so obsessed with this? He just added a black Solid Color layer then masked it around the boy. It's kindergarten Photoshop.
He used the Shift key of course. https://www.damiensymonds.net/2010/10/shift-while-brushing.html
You're lucky. Every calibrator on the market will run on that computer.
Get the best one you can afford.
https://www.damiensymonds.net/which-monitor-calibrator-to-buy/