
The first thing I noticed is that since he has his hand raise foreward (nice hand by the way! And you did a good job on the foreshortening) is that his upper torso is going to be twisting slightly, or if anything, he is at a 3/4 view. So his shoulder was appearing at a frontal view. You need some dimension on his shoulder to show it is going foreward with the forearm. His thumb could be slightly longer.
For his arm further back, it should taper (also because it is going back into perspective, so maybe a bit more dramatically depending on what effect you want).
His head is too small (gotta fit those brains in somewhere) a good way to indicate head size is if you draw the face first - the eyes are in the midline of the head. Trust me - it feels weird at first, feeling like drawing the head too big - but the eyes are in the middle of the head.
Even though his legs are ending offscreen, sometimes its better to sketch where they would go then crop it after. Right now his legs appear to be one constant size when they should taper (even with baggy pants there will be a slight taper) into the knee. He is also standing a bit wide legged for a strong stance.
I hope that isn't too nitpicky, its looking good already! Good luck and hope this helps.