pay it. then tell them to send a letter to the credit bureau(s) to remove/expunge it.
Not an expert but I believe that would work.
-100000
First of all, as others mentioned, it should be removed as he was under age.
But even if it was legitimate, it's from 3 years ago. Paying it now, it would report paid this month. Effectively, only bring the date of a negative item to the current month and hurting the score more.
Then falling off in another 7 years.