Anatomy: There are a couple of isolated problems with the anatomy (e.g. narrow shoulders, short and heavy arm without a clearly defined wrist or elbow), but I think the most important thing to work on is the entire pose. Right now it's doesn't read very well; I'm not sure if he's holding the chair, or if his arms are just dangling down. Or whether he's sitting with a straight back or slumped. Or how the head and the shoulders are positioned. Perhaps finding a good reference photo would help a lot. (And if you like, I could redline it.)
Shading: There are three things you could do to improve the shading. The most important one is to be consistent with your light source. For example, the rear leg of the chair casts a shadow that suggests there's a light source behind the character. The highlight on the arm looks like there's light coming from the side. The rest of the body has only a sliver of shadow, suggesting there's another light source just to the right of the viewer.
Another thing you could do is add more cast shadows. For example, his left leg would cast a shadow on the tail and part of the other leg. Or perhaps his arm would cast one on the chair and on his body. Same thing with his trousers and his feet.
Also, don't be afraid to add more contrast. Some deeper shadows will give the drawing more punch.
Composition: A good rule is not to let your subject touch the edge. So you could either add more room at the bottom, or cut off a good chunk. Another rule is the "rule of thirds"; instead of centering everything, move it to the left or right 1/3rd of the paper. Or to the top or bottom 1/3rd. Also, pay attention to the perspective when placing objects in a scene. The vanishing points of the chair and the room suggest a very high horizon, so the viewer is looking down. This also means the foreshortening of the character will be different (for example you'd have to foreshorten his shins instead of his thighs.)
I hope I could be of help. :)