You've misplaced the man in the background's bicep. With his arm turned like that, the bicep would be facing outwards, you'd see the triceps closer to his body,and you'd see the Coracobrachialis attaching the arm bone to, well, roughly the collar bone.
Same thing goes with the other arm. The bicep would be facing forward and outward slightly. Not inward. As mentioned above, the wristband he's wearing also needs to have both of it's lines on a downward curve, because his forearm would be coming towards us slightly.
Just because you're good doesn't mean you shouldn't improve. Besides, I could have also critiqued his monotone shading. What he should really do is make shadows move to cools and highlights to a yellow or some other actual light source. He's not that great. Good for 17, yes, and respectable within his knowledge, but if he doesn't progress at all he's just a poser.
How the hell is he a poser? but still its a critique and all and i really agree with with you and all,but he could easiley beat most of the older artists on deviantart. But its useless to critique on someone who is popular because you have a low chance of getting your critique to them.
Well monotone shading is pretty usual on anime\manga style, not everyone likes to shade using alot of colours, some people like it simple just to define forms, sometimes its cool to move away from realism, anyways critique is always good if you want to improve, your 17 and your in the right track just keep drawing everyday.