Collection calls to me for my son

My son is 51 years old; in 2005 he stayed with me 3 mos following a divorce and while searching for work after relocating to the state where I live following my retirement; apparently because he stayed with me this short period his name is included on my credit report in some fashion and they are trying to get payment from me for an old credit card debt that was written off 2 or 3 years ago. I asked them not to call me. I'm not in contact with my son, I have nothing to do with his debts and I cannot pay them anything. This debt was written off before he was approved for a mortgage and bought a home. He made payments to a collection agency on this credit card and then found out from the bank that they had written it off and any monies collected was kept by the collection agency and did not apply to the amount written off on their record. What can I do to stop this? I'm 71 and have no means to pay anything on this. I will contact the FDCPA mentioned. I contacted the credit reporting agencies and was told they cannot remove my son from my record even tho we never had any connected credit dealings of any kind.