The "equating test" is when a selected school is asked to get their 11+ cohort to sit one of last year's actual papers (2006). Different schools are selected each year on a rotation basis.
It is used to judge the difficulty of the two tests when compared "like for like", and thus the performance of the children this year versus last year. It helps to ensure that the test remains rigorous from year to year and probably yields a myriad of statistics.
Although I can understand your son being a bit fed up about it, his personal score is not important in this test. It is however an essential part of the process in Bucks so perhaps you can suggest to him that he might think of it as an honour to have been selected?