Builder not handing over the flat although full amount paid

Hi, I purchased a flat from a builder (not reputed builder. he is a newbie). I booked it in 2015 and paid the down payment via online transfer. The rest I paid via loan from DHFL home loan on 2016 December. Since then he is saying he will complete and give the handover soon. But he has not completed the house yet. There are still lot of work to be done. While buying the house, he told the total price is 31 lakhs including the parking and other charges. The duty and registration was paid by me. Last week he called me and asked to pay 1.2 lakhs against the service tax. He forgot to charge the service tax and someone from the society has paid the service tax on his own and thats why he is getting the notice from IT people. What I don't understand is he is claiming 15% tax on 25% of the entire cost. The GST was introduced after I purchased the flat. My question is what are the legal options available for me and am I liable of that service tax he is asking for.