starting from the top working down.
not going into detail on the face as things have been mentioned, so moving further down
neck is too thick for his body
seems like youre better at drawing the shoulder thats on the right side of the drawing, but the line that goes from the shoulder to the base of the neck is too horizontal, theres really no muscle structure whatsoever, the arms too long on the top half, and the forearm is too short, elbows are way twisted looking, on the front view you shouldnt even be able to see them standing like that, and actually taking a second look the are are too long for his current torso but more on that when i get to the torso itself. hands are not bad, they could use some work, but all in all better than most people tend to draw them.
ok, on to the torso area, its too short. stand in front of the mirror and note where your elbow joint is in respect to your waist/beltline* (*if you wear your pants right that is, you know, pulled up, on your waist) if that were to be fixed the arms overall length would be ok aside from the upper/lower arm proportion issue.
legs are too thin, a bit too short, he has no hips, feet look like theyre too short, also perspective is off, to see the top that much you would need a higher viewpoint. its almost as if when you look at it from top to bottom you go from a slightly lower than eye level viewpoint to about a foot above his head by the time you hit the feet, and as has been mentioned, knees are quite knobby.
back view roundup, no hips/ass problem persists but its consistent with the front view which is a good thing, at least you can draw the same thing twice, other than the for mentioned feet issue, the back view is a more consistent viewpoint/perspective.
another note, in both back and fron view the arms come in front of the back and chest, if his arms overlap his chest then in the back view hit lats would overlap the inside of the arm slightly, same with the front if his arms are back, it makes his chest look very shallow and flat.
hope this helps and i didnt seem like a dick, just trying to help you improve