Well I'm a reasonably big guy so don't have that problem that much. What I do try sometime with a more junior in a static practice like this, is lock down a bit and have the junior see what movement (internal, shifting weight, relaxation, etc) will start to impact me. Sometime they are amazed at how relaxing and dropping center can impact me so much. What I suggest is, instead of basically fighting each other, ask him to do what he does and tell him you are going to try changes in your body and have him tell you how affects him. If you still can't move him, ask him to lighten up a bit and then see the results. Think this will make him a partner in your learning and not an opponent.
Good luck on this,
Thanks Bruce, sounds like a good plan.