those are photos placed into vector art backgrounds.
As for the effect, if it took him a year of trial-and-error, I'm not even going to hazard a guess.
But if I did have to guess I would say it looks like several steps for each photo. Hair/Skin/Clothes are all treated wtih different techniques so masking will be your first challenge. It also looks like he sometimes includes lipstick or heavy makeup in the clothes processing.
And tbh, some of it just looks like bad airbrushing like you sometimes see when there is absolutely no detail left in a models face. Which is not to insult his skills. I'm sure that's the look he's after, I'm just making the comparison to say... the recent shot of Carmen Electra on the cover of cosmo. In that case it's supposed to be serious and realistic where in his case it's perfectly acceptable because it is part of a stylized image.
As far as the clothes... It wouldn't surprise me if he used poser to render the clothes.