Buyer Not Transfering Ownership of Bike, Not Receving or Answering Calls.

I recently sold my brothers bike, exactly one month ago, the buyer paid me in cash, i handed over the form 29, 30 and RC Book and Tax paid to him and took his signature on a sale agreement copy along with the witness signature. He promised that he will immediately transfer ownership in his name. i took a photo of his aadhar copy, where his permanent address is in other part of the state and he mentioned that he is staying in bangalore, and will provide his local address. I also collected the photo of the Drivers licences of the witness. From next day he has not been responding to my whatsapp, after checking the details in vahan i find that he has not done anything for a month. I just have the vehicle sate agreement signed by him and witness. It is a four page agreement clearly mentioning that he has to do the transfer within 15 days. The vehicle sale agreement is not notarized. I am not able to contact him, he does not answer my call at all. How do i protect my brother from this mistake.