L&do freehold conversion based on registered sale agreement

Property transferred through registered sale agreement in 2005. Payment to seller and taxes paid in full by the buyer at the time of execution of sale agreement. No sale deed executed at that time. Possession of property held by the client (buyer’s son) since the date of execution of sale agreement. Property in Delhi and in original condition and no new encroachments or construction. Buyer deceased. Buyer's husband deceased. Both died intestate and survived by 6 legal heirs. All legal heirs executed relinquishment deeds in favour of the client. Sale agreement contains a clause whereby seller agreed to execute sale deed in future. There is no GPA or SPA in the favour of the client. Property in question is L&DO property and is on lease. Can a sale deed be executed between the original seller and the client (original buyer's son) after 16 years of execution of sale agreement, full payment of property consideration to seller and hand-over of possession? How must the client get ownership?