Hard step the shorter iron shafts to reduce spin and add loft to them to add launch. The added loft will add launch and spin as the hard stepped shafts would stiffen the tip and cause a more penetrating ball flight adding total distance. This would help close the 20 yard gap and get it closer to the ideal 15 yard gap.
Callaway Pro are strong lofted irons, so his 5 iron will be playing more like a 3 and creating the gap of 20 yards.
You could reduce the length of his 5 and 4 irons slightly to reduce their distance, or slightly up the length of his p through 6 to make up for this.
The solution to the wide distance difference, between the irons of concern. This situation requires a slight change to the Loft and a slight change the Length of the affected clubs.