It's from a popular cover of Can't Take My Eyes Off You: https://youtu.be/GIG38MD_l3k?si=y7f6D81MRsf7fG2G&t=137

When he reaches those high notes (timestamp linked), is he pulling his chest voice up, or is this is actually a really refined mix voice?

I've been trying to learn how to sing higher notes, but when I use mix voice, the sound is nowhere near as full/strong as how this guy sounds. I'm wondering if it's just a matter of practice to get there, or if actually he's belting.

Definitely what I'd call a chest-dominant mix! Getting a fuller, beltier sound in mix often comes down to (1) experience & comfort mixing, (2) vowel choice, (3) body engagement, and (4) other resonance strategies like adjusting the mouth shape, tongue position, etc. 

See Abram's lessons on mixing and belting for exercises and tips!