Has he been doing any overtime or worked more hours than normal during any weeks/months? Or maybe received a bonus or extra shift pay?
They base the student loan payments on your income each month, so if he happens to have earned more than the minimum threshold in any week or month, they will deduct the student loan amount accordingly.
For example, I graduated uni in 2005, but didn't earn a salary of more than 15K until 2010.
However, one month, I had several weeks' worth of Sunday shift allowance paid out in one pay check, which took me just above the minimum amount for student loan deductions... so I had to pay £22 of my student loan that month. I also had £2 deducted another month due to working some overtime.... so in those 5 years, I only paid off £24 of my student loan
.
Even now that I am earning over 15K, my student loan deduction amount changes each month... depending on how much weekend shift pay I've claimed.