The flannel seems a little big for him (the sleeves look long), so maybe it’s Sam’s? The cuff comes out much farther than it usually does on him.
He was originally wearing Bobby’s clothes, I presume, because his dug out of his own grave clothes were kind of shot when he goes to meet Sam but then he’s changed again after he and Sam hug it out.
After I made this, it was really bugging me because I was pretty convinced that brown jacket was Sam’s too (it’s buttoned back in the first gifs up there because it’s too long, I guess) and lo and behold
Sam’s corduroy jacket, which we see a few times throughout the series, but this one in particular was 3.10. And we can see it has the fleece lining.
which we can see is in the jacket Dean’s wearing, as the edge flaps in the wind, in 4.01 iafter Sam and Dean leave the demon cafe
And while we’re at it, that’s definitely Sam’s shirt too. Sam wore it in 2.04
The diamond buttons (I know the colors don’t look the same, but that’s because of my poor giffing skills shhhh)
There, curiosity and deep obsession with everything Dean Winchester satisfied.