1.  File a Civil Suit for declaration and cancellation of patta, in the local civil court, by following due procedures of law and with supporting documentary evidences and witnesses.

2.  IF you suspect some forgery or cheating THEN

a)  You can file a Police FIR, requesting investigation and charge-sheet, for offences like cheating, intimidation, fraud, breach of trust etc.... against the person/s, supported with all relevant supporting documents.

3. IF the Police does not take action, THEN you can file private criminal case u/s 156(3) Cr.P.C., in the local Magistrates court, seeking directions to the Police to investigate and file charge sheet. The said Person/s may come down for settlement.

1)approach the revenue authorities to remove patta granted in son name 

2) enclose copy of sale deed by which your father purchased the land 

3) if they refuse take legal proceedings to set aside mutation of land done in name of son 

File FIR, patta is illegally issued in his name.

Hi

Meetup with the concerned VAO / Tahsildhar and share your case with them.

Enclose a copy of Sale Deed which your father bought from old lady.

Question the mutation carried out in her son's name.

Issue notice to that effect questioning the Mutation in her son's name.

If they do not incorporate your father's name file case against the erring officials also seek removal of her son's name from the Revenue Records.

Which was earliest, patta or sale deed?

Have you applied for encumbrance?

G.Rajaganapathy 

Its is a illegal thing you can file a complaint against the same

Patta is a revenue record, and by itself is not a document of title. Does your father's name appear in the joint patta? You may verify online in TN. If so, you may apply for an individual patta in your father's name. Depending on the response from the Revenue Dept. officials, you make take further steps.

A Patta is registered by the Tehsildar of that particular area.To get your name registered under the Patta, you have to make an application to the Tehsildar, requesting him to get your name registered.as property bought by you from soul owner enclosed certifiied copy of sale deed. He could after examining the application either accept the application or will reject it. incase if he reject you have to challenge before Civil court. 

You need to file suit against the son and Tehsildar of your area for mandatory injunction for updating your father name in patta record.

Since the property was purchased by your father by a registered deed, he may apply for transfer of patta to his name and wait for the authorities clarifications if any.

If the Thasildar  is not cooperation then you may file a suit seeking directions to the Tehsildar on this.

1. As per the information mentioned in the present query, makes it clear that your dad has legal right over the land irrespective of the pattaa being on someone else’s name as he had got it registered much prior to this new pattaa as of now.
2. You should first inspect the public register which contains all the information of property and then see who has done approved it.
3. Them make a representation to the department seeking answer to your query.
4. If they don’t reply then approach the Hon’ble High Court under Writ Jurisdiction seeking immediate removal of the newly pattaaa name and direction for the staff to clarify.
5. After this once you get the order then proceed for FIR to make your case more strong.