Well, you know how becoming a writer changes how you read? Forever?
Everyone's like that. You've just got to work out what exactly he sees and what are his blind spots. Because just as you can't turn off the imp that points out that that use of the progressive voice was unnecessarily wordy, the character can't turn off his knowledge of -- whatever it is he knows. A farmer will notice the plowing, the ditching, the crops. A soldier will notice how defensible a building it, and who moves like he can fight, and what would be the best place in a tavern if a brawl broke out (and insist on sitting there, too). The fencer will snark about the swordplay in a movie, and the costumer about the clothes and hairdos. And in a society with nobles and commoners, everyone's going to determine social rank on a glance.
Which does have the advantage of slithering in local color -- when you do it.
It's one of the hardest things to do because it's unrelenting, and the temptation to reach for the obvious thing -- what you would notice -- is always at hand. Plus, the added difficulty of when you realize that your character won't notice what he's got to notice if the readers are to think that the conclusion makes sense.
Acutely sensitive characters, or characters not that different from your own POV, help, but it's always something to wrestle with.