In 2008 My XXXX son left for college at university XXXX. We rented him an apartment at XXXX XXXX XXXX XXXX XXXX. He was a XXXX so I rented the apartment I my name. # months int the lease they stopped him on the way in and advised him that he was required to sign an addendum to the lease or his rent would go up. He was hesitant but they bullied him into signing. He moved out at the end of the XXXX year as scheduled. 
Now they are reporting to both my and his credit that we owe {$6100.00} for a defaulted lease. I called them and they said that because he signed the addendum he agreed to extend the lease for another year. 
First of all, he was not authorized to sign an addendum as he was a XXXX and was not the lessee, but the sublet. Second, They have been actively trying to collect on this for XXXX years even though I have repeatedly advised them that it is not a valid debt. I still receive aggressive collection calls on this. Third, he is unable to secure Student financial aid as a result and he is in his XXXX year of XXXX school. 
Please advise the steps I must take to eliminate this.