I assume you did something like this and got the list, right
No, I am not sure I am ever going to bother to return the books. If they dont tell me what the books are, I dont think I have a duty to pay the fines or return the books.
He checked out the books. Just because you don't like the policy (and I agree) does not mean keeping them is a good response. The only potential effect is hurting your son's record. May even go on his credit rating someday after the fine grows over time and he becomes 18. (Some articles were posted several months ago about libraries reporting to credit rating companies) Sure legally he might not be responsible due to his age but why create a mess for him? Just have him find out what books are out in your presence, have him return them, the cancel his card or whatever.