Bought a flat from GPA holder and facing issue from Owner

Hi, Last year i had purchased a flat under owner share. Initially i was totally unaware about the person with whom i am dealing is a GPA holder. Son is the owner of the Flat and Father is GPA holder. GPA is registered. I even did registration last year in the month of November with the GPA holder. The problem started when son came to about it. He stopped the construction work (only my flat) by discussing with builder. Upon discussion with GPA holder construction work started again. Now the issue is, construction is completed and Son has taken the key with him and not handing over to me. When asked he is saying the GPA is already cancelled and its not valid. He also saying that i can check Book 3. GPA holder says that how its possible, for cancellation of GPA, the owner should send him the notice and then after his signature only GPA can be cancelled. 1. How/where do I check if GPA is cancelled or not? 2. Whether my purchase is legal? 3. GPA holder says that he will go with the owner legally. - If gone legally how much time it will happen to sort out the issue? - And will it be in my favour. 4. Lastly, should I take my complete money back and just come out this Father & son issue. Please suggest. Thank you