Buyer is not transferring ownership of bike

I sold my bike last year but the buyer is not transferring the ownership of the bike. I have called him many many times but there is no response. As a proof of sale, I have a blank paper with his name and address confirming the bike purchase.
I am unable to register any vehicle under my name now as the RTO says that you already have a vehicle registered under your name. They are asking for additional tax.

I am from Hyderabad. What are the options I have?