If I am understanding correctly you are looking to explain that he is falling further behind his peers rather than catching up - right? And that even if he is only slightly further behind than he was 4 years earlier that is still significant.
The best way I have seen it worded in layman terms - you need to see more than a year's progress in a year's time or they will never catch up.
His peers are presumably gaining each year - if he gains an equal amount he will always remain behind. If he makes less than a year's progess in a year's time he will continue to lose more ground. Each year's loss may be incremental but each of those incremental losses add up to being further and further behind his peers.
Proper remediation CAN close the gap. If he is further behind his peers today than he was 4 years ago the remediation has not been successful. For example my DD went from decoding below grade level to decoding at grade level to decoding above grade level during that same time period. That's demonstrative of successful remediation.
