I'd suggest that he should take this up with his payroll department and/or visit/phone the Revenue Commissioners to clarify the applicable treatment.
Prior to giving you my answer above, I checked an Income tax book which indicated the outlined treatment- you are charged using the rates/rules which applied in the period to which the income refers, no matter when it was received. In the book, an example was given of a salesman receiving commission in March 2008 which referred to the calendar year 2007. Tax was charged using 2007 rules/ rates.
It could be that some time limit applies which the book doesn't indicate but as it would likely be to his advantage to get clarity I'd recommend further query!