how a father responsible for sons debts?

Sir Namaste. My son who is not living with us obtained credit cards and used them without proper payments. at one stage the HDFC credit card recovery agents came to me and offered onetime settlement with some amount which I am paid because the payment is within my capacity and I am in service at that time. I retired in 2013 and receiving a pension of Rs 1,669/- P.M. In Feb/March 2015 some people came to my house and demanded payment of credit card dues of my son whch he took in March/April 2009 with limit of Rs 33,000/- and used 4 moths to the said limit and paid Rs 9300/- in the due course. As per the statement of the said card the due amount is more than 80,000/- upto Nov 2011. I told them that my son is not living with us and we don't have any contacts with him. At one stage in the middle of the March 2015 they again came to my hous and I told them that , as the card limit is 33,000/- and the bank already received Rs 9,300/- the due will be about 23,000/- and as a good gesture I agreed to pay upto 15000/- as one time settlement . They said they will inform the same to the bank and let me know the answer. But they have not come to me upto 3rd May 2015. On 4th May 2015 some other people said to be the agents of the said bank came and demanded the recovery by saying that the bank not accepted my offer. I told them that being a retired person I cannot afford any more amount. One of the agaent started abusing me and warning me that he will lift my car and proceed against me in court and reover lakhs of rupees from me. I told them that they have not taken any permission from me before issuuig the credit card in march 2009 and after a period of 6 years you came to me and demanding the dues of my son. They donot care about my submission and started shouting in front of my house. I could not understand how these people reacting on the issue for which I dont have any information and binding. Please advice what I should do. Sarveswarudu.Duvvuri 9030529301