You don't vacate the premise on the given deadline.
You ask him to proceed through court of law to evict you.
If he is the real owner then he will certainly file an eviction suit, if he is not then he may come for a compromise with you.
If he has shown you the possession letter issued by the builder on his name, then he should be having the registered sale deed also, hence you may scrutinise the possession letter i.e., to whom it was issued, if it was his father whether he is the actual son of his father and if yes, does he have any power of attorney deed from his father to deal with this rental agreement as well as to collect the rent.
You can also issue a legal notice questioning his authority over the property on the above lines.