1. Without a registered sale deed no title passes from your seller.
2. So before purchase first ask the sale deed to be registered in the name of the father.
3. once they do then make a sale deed with the father only.
4. Since the BDA has not made the sale deed as yet, the gift by father to his son does not make any clear title in the property and hence even if the BDA now makes sale deed in the name of the father, your purchase from his son will not hold good.
In other words unless the conditions as stated above is fulfilled do not buy this property.