I really do not know the skill set required for these folks nor the criteria for placing them in a wage scale. This is how you need to approach this.
1) Understand the wage scale, is it based solely on time in service, hopefully not, or based strictly on skill set then performance based, preferred method.
2) Once the wage scale is understood this may allow for a better understanding of why this man is paid more or why he is overpaid.
3) It his skill set dictates that he should be paid more perform a special performance review with the the co-workers. Set goals for them to strive for and a time period to accomplish these goals. If the goals are met it will be a win-win situation both for the co-worker and the company in a monetary sense. Have HR clearance for this. If not then truly the only problem you have is to hold this co-worker accountable for his pay scale. Being paid more means his performance in all disciplines exceed all expectations, as this mistake is not yours this is the best way to handle this.
In the past I have been faced with these types of situations and have handled it in this manner. In the end everyone benefited. At times it meant that some were replaced but for the most part this was a good team building exercise and paid dividends. Handled correctly this will provide an outstanding outcome.
Make sure these goals are SMART Goals.
S= Specific
M= Measurable
A= Attainable
R= Relevant
T= Timely
Hope this helps Brian.
