This is an amazing picture. Your shading is really good. It does look a little stiff, but thats ok. One step at a time. Maybe what can help your stiffness is to apply limpness. Instead of the arms pointing straight out, away from the body, draw them bent some more, and closer to the torso. If you sit in front of a mirror like this, and let your arms naturally hang loose, they would fall closer, and be against the wall some more. His right arm looks pretty good, the one next to his tail. But the other one looks like he is forcing it out, away from himself. although someone could do this, I think it would help deplete the stiffness if his arm was more limp. Still really good and I love the expression on his face. Your shadows are well placed as well, so I think you have those down pretty good.