OK, first things first, you need to clean up your line art. You can argue that it's in your style but it just looks rushed and unfinished. Having a clean sketch/linework will definitely make your image look better.
Secondly, the deltoriod (shoulder muscle) on his left arm (our right) looks way to big for the kind of character you are going for. It also doesn't match the other arm.
Lastly, I strongly suggest not using the gradient tool in this way but actually shading it for your self. It just looks too airbrushed and smooth to look natural and any user of photoshop could pick it out a mile away.
Keep working on your art though, I can tell you really have a lot of passion for it!
