Do you have insight into the other dudes performance agreement/duties/additional projects? Do you have insight into the feedback he has received?
If he is not up to par maybe the management have a plan to deal with it that you are not aware of. Management might not have felt they could drop his pay when they took over. However maybe it's written into his performance agreement that he needs to meet XYZ goals or his rate will go down. Maybe they want your pay to be more equal, they just can't do it right away without attracting a lawsuit.