The buyer seems to have purchased the property by paying full consideration but instead of getting a registered sale deed executed in his favor, he has simply got a GPA deed on his name. This has been a routine practice of many people in Bangalore and surroundings for a very long time. However this GPA do not confer title on the buyer. But the GPA can sell the property unto himself as a vendor to the seller and it is valid, So he need not take the consent of the original vendor but the hidden danger is that the registrar would ask him to signature of the original owner now to confirm that he is alive.
For claiming ancestral property share, the minor should have done it within three years from the date of becoming major by age.