I opened these up to view them at 2048 px
1, I don't mind it being under exposed, it's just noisy and a bit oof. the focus lands about a foot in front of his face, this is why his shoes are in focus, but not his face.
2, cool lighting, dof is a bit too shallow. his mouth is oof, his front eye is barely in focus, I'd say stop down a bit so everything is sharp, you don't have a background that you're trying to separate him from, so why such a shallow dof?
3, the crop is weird, you cut off just the ends of his hands and left a bit too much head room for my liking. I'm not sure if you're intentionally trying to make this photo look like film, but there's dust/hair going on in the photo..? I'm not even sure, if that's what you're going for, I won't give you any creative direction, but I think it could have been executed differently.
4, slightly oof. if you look at the wall he's leaning against, you can see that everything is sharp about a foot in front of him. his eyes/face are not in focus. only off by a little though. samething with the composition, you cut him right at the elbows which is awkward to me
5, pretty sharp, I like the composition. there's plenty you could have changed while shooting that photo to make it better, but for what you have, it's pretty good.
6, my favorite of the set. it's the sharpest photo of all of them, the composition is good. I'm not a huge fan of the typical rail road picture, but it's alright here.
I'd agree with mtn4mike. retouch his skin a tad bit. shooting portraits at 1600 iso isn't preferred, but sometimes necessary when you don't have enough light.. I would try to limit how far you push your iso next time, even with your 5d2.
great job on these, keep shooting!