I tried to do a quick sketch over it and see what could be done.
I don't know anything about Star Wars, but from how stocky he looks in your image, I just imagined he was a beefcake. Also, I am by no means an anatomy expert, so pardon if it looks cartoonish. I've seen some here color out the muscle groups, maybe that person will stop by.
The big thing here is proportion and foreshortening. His shoulders are wide, but his left arm is thin and too far out. Bring that whole arm in some, thicken it, and get a little more exaggerated with the ribbing in his sleeve to show more direction and depth. We want it to look like that elbow is going back in space, and we want to keep the length of the upper arm equal to the length of the forearm. Right now, the upper arm looks shorter than the forearm.
For his right arm and hand, it's again more foreshortening. The hand and forearm are closer to us in space, so we want them to be larger—they're too small right now. And the curve of the ribbing on the forearm can be a little deeper too, just to push that illusion of perspective. (I changed the angle of your hand a little in my sketch, but that's not necessarily a suggestion, that's just me not into hands today, lol). You may want to add some visible darks and red highlight in that hand, too. It's pitch black and no light escapes it.
That's all I see, though. I hope that helps!
