That's a tough one! I can tell you that if that were an animation school exercise, several weeks would be spent on it, so we can hardly expect it to be perfect after the first few hours. It's good that you are animating again and this is a good start.
Dhar and Ruscular are homing in on a good point, that you want to create poses that really suggest the forces at work... gravity, his legs, his arms, his back. Grabbing the box by the sides is the easiest to make a CG character do but would be ineffective for a real person. I'd recommend acting this manuver out with an actual heavy box and see what you really have to do to lift it. Better yet, get someone else to injure themselves while you watch.
It just so happens that in another thread (post #64)we were discussing someone else's heavy lift and why it didn't look convincing. It has some similar issues.