Like EricShawn says, this is good sculpting, but I do believe critics are good to get tho. And I bet you can get lost even if working with reference material, because when you look at a model for to long it tends to just blend in to your brain, and it’s not to easy to spot things that can be corrected to get it better. I was not going to write that much as I did bellow, but stuff just kept popping out 
It depends on how realistic you want it to be, and if it’s a human and not an alien or foreign sort of human, but some human anatomy is of:
The first thing I noticed was his thumb, it should be twisted a little bit, look at your thumb, you hopefully got one right infront of you. The second thing is his ankle area, it’s kinda squarish, but could be that your foot needs to be wider. And the knee, it’s not two “half moons”, but one “fullmoon” with wrinkles 
His quadriceps ( upper leg front ) looks off, atleast not right. The back of the upperleg seems to have gotten the same treatment as the lower legs, but the muscles in the upper leg differ from the lower. Outside his abs you’ve given him a couple of more “bricks”, instead of just the muscles who are covering the ribs. His triceps are attached to the lower arm muscle, but it should really just stop at the elbow.
This probably seems like alot, but it’s only small adjustments really, go to flickr. Search for some body builder images to see how the muscles looks when they are defined.
EDIT: Since i’m working out a couple of hours a week and into fitness, I should probably learn the names of these muscles and stuff. But I bet it’s just easier to explain with other words since a normal person dont really study muscle names anyway 