No particular format is there
The doctor has to simply state on his letterhead that you are alive
He has to sign the letter and put his stamp on it
To such letter your photo ID (preferably Indian proof like PAN or Aadhaar card) can be attached (preferably a certified true copy duly certified by Indian Embassy or Consulate) and the doctor can further write that you showed him the photo ID and the person in it and the person before him is the same.
Not necessary that such a live certificate be given by only a doctor. Any person like a public notary or any person from the office of Indian embassy or consulate(preferably) can also give this certificate
Additionally an certificate can be obtained from the local Municipal Corporation in your residence city in Chennai stating that no records are found on the death register for a person named so and so (that is you). This should allay all doubts in the mind of the sub registrar
In my view the sub registrar is doing the right thing by taking adequate protection which is really a good thing in view of the scams by which properties of NRIs who are dead are sold using POA by unscrupulous people