I had the same issue, but I have a fix for this. Lemme find where I posted it...
Ahh, here we go:
...I thought it might be a bit of platic from the tree that hadn't been cut of properly, or a clearance issue, but I couldn't find anything. I tried forcing it but it wouldn't budge and I felt that I might be stressing some plastic somewhere. I thought, "Maybe they misassembled something?", so I took the arm apart, turned it around and tada! Perfect.
So if anyone else has this issue, just unscrew the two screws in the middle of the back of his shoulder. Loosen the top shoulder screw (don't need to fully remove it). Then take off his arm, rotate the black shoulder joint bit 180 degrees, and put the arm back on.