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Well, I might give you a redline if I can find my tablet...

The main problem, in my opinion, is the arm.

The head is alright, the neck is a bit thin, but excusable...

But the shoulders and the arms definitely do not match up. I think the shoulders are generally the right size, but his arms are wayyyy too skinny. They need to be beefed up. Add biceps and all those arm muscles that he seems to be missing.

The side of his torso, on (our) right side looks too curved in as well. It doesn't really match with the curve of the other side of the torso.

Like, the torso should be curved more like this (not as exaggerated, though):
( (
rather than what you did, which is like this:
| (

But yeah. Fix up that arm and it'll improve the picture A LOT.

PS - his chest/n****e areas look a bit too far apart as well. Maybe next time use a reference?

If in the planning/sketching stages you tried to define his skeletal structure a bit more, you might more sucessfully capture to skinny boy look. (rib cage, anyone?) big pecs don't make sense on a very thin character. If his body mass is that low, the outer shape of the pectorals wouldn't have that much definition.

i think his collar bones are too short/high as well. They are supposed to be connected to the shoulders a bit more.

really, just looking at or sketching any skeleton would give you a much better idea of how things are supposed to fit together, and once you can imagine the bones, you can start to picture how all the flesh sits on them as well.