Great job completing the assignment - the detail work looks good to me, however, I think you could use more practice with proportions and showing depth in your shading. He looks a little flat, because everything appears to be shaded in the same value range. I quickly added some dark values over your drawing, I hope you can see the difference. By adding shadows around his torso and legs, it helps them fade out of attention, drawing the viewer's eye to the centerpiece - the man's face and flexed arms. Since I don't know what the original lighting was, I mostly guessed on it, but I hope it helps you understand what I'm saying. I struggled with my shading when I first started too, I always made it way too soft, so I had to keep in mind to really push the dark areas and focus on the LARGE shadow shapes instead of getting caught up on details. Get those large masses down first, and your drawing will feel really solid. Then, you can start putting in details (such as individual muscles, veins, and skin folds/creases).