It's pretty easy to replicate. Just open an image in photoshop, go to Image > Mode > Indexed Color. Drop the number of colors low -- single digits for simple art, teens/low 20s for more complex -- and set the dithering to diffusion. Then play around with the diffusion amount and settings until you get it where you like it (you have to undo the Mode change or change back-and-forth between Indexed and RGB/CMYK to make further adjustments). There's probably some filters he throws on top to enhance the effect. Indexed Color Mode can be a lot of fun to play around with. Make the image really small while in Indexed Color Mode and then use Percent instead of Pixels/Inches to enlarge it again to give it the classic 8/16-bit style pixels as well.