You could test whether the problem lies in the granularity with which you or he can assess the level of pressure applied by trying a variant: Place your hand on his shoulder, elbow down, and attempt to hold your arm stiff while he pulls down on your elbow with both his hands. Then, in the same position, attempt to point or reach behind him while he does the same thing.
Since he’ll have to exert more effort in both cases, the difference in effort should be more apparent.
You could test whether the problem lies in the granularity with which you or he can assess the level of pressure applied by trying a variant: Place your hand on his shoulder, elbow down, and attempt to hold your arm stiff while he pulls down on your elbow with both his hands. Then, in the same position, attempt to point or reach behind him while he does the same thing.
Since he’ll have to exert more effort in both cases, the difference in effort should be more apparent.