OK, let me start by saying that I'm a doctor, but I'm not your doctor. So nothing that I write should be considered medical advice. (Not trying to be a jerk; just making clear that I'm not establishing a doctor-pt relationship with you. Sometimes folks get the wrong impression when a doctor gives them feedback over the internet.)
What you've described sounds like an injury to the musculocutaneous nerve in that arm. The musculocutaneous nerve controls the muscles of the anterior compartment of the arm (the biceps, the brachialis, and another little muscle no one has ever heard of), and provides sensation to the area delineated in the figure.
Based on what you've written, it's impossible to know whether this injury was precipitated by the original trauma, the surgery itself (nerve damage is a known complication of shoulder procedures), or just prolonged post-op positioning (ie, being in a sling for a long time).
What would I do if it were me? I'd make sure to keep an open line of communication with my doctor. If he's right about it being sling-related, the strength and sensation should improve with time. If improvement didn't occur in a timely fashion, I'd ask him to consider whether I might have sustained a nerve injury at the time of surgery, and what if anything could be done to treat it. I would be nonconfrontational and matter-of-fact (because I know he's less likely to become defensive if I don't appear to be on the offensive). If I felt he was stonewalling me or being less-than-upfront, I would get a second opinion from a neurologist (as they can do special testing to determine whether a nerve is intact).
Best of luck. Keep us posted.