Student loans are not considered qualified higher education expenses for a Coverdell ESA distribution under the current law. You son cannot claim the repayment as an education expense to offset a Coverdell ESA distribution.
The IRS has stated that if you pay for education expenses with money from a loan, you take can use the education expenses in the year you pay the expenses, not in the year you get a student loan or the year you repay the student loan.