Hello.
In the way of critique, I'm going to focus on the neck-head area.
In general your heads are to big. They make your characters appear younger and more childlike. In the newest picture, this could be fixed by making the area at the back of the head smaller. That would also fix the problem that the head looks too far back on the shoulders.
Necks are like trees. The base of the trunk is never smaller then the top of the trunk. It should be larger at the bottom and taper towards the top. It's possible that someone really bulky could have a neck that doesn't taper, but this only happens occasionally.
It's good to see you including the collar bones and the muscles of the neck, as this shows that you have some knowledge of anatomy.
Just quickly onto shoulders, in the second last picture the shoulders slope downwards too much. Shoulders should be inline with the collar bones and slightly (SLIGHTLY) slope if they are slouching like your character. At the moment he looks like both shoulders have been dislocated. If you look at this picture of Justin Timberlake you can see the way the shoulders should slope. (Possibly half a shoulderlength down from neck, while yours is a whole shoulderlength)
I hope this helps and I wasn't to harsh, because you have potential. You just need to practice. :>
If you have ANY questions, feel free to ask.
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