Relinquishment of house property

Property purchased by father and son by following contribution: Father's personal saving- Rs. 12.00 lac Bank Loan taken by son with father as co applicant: Rs. 6.00 lac. As per Deed, property is jointly owned by father and son, however share of ownership is not mentioned in the deed. After repaying the loan taken from bank, son wants to transfer his portion of share to father> 1) Whether to execute deed of Relinquishment or Gift deed? 2) Whether payment of registration charge and stamp duty depend on proportionate value of property (son's share on property)to be transferrred? 3) In case no explicit mention of son's share in property, whetehr share to be determined as per proportion of finance ( i.e. 6.00 lac of total 18.00 lac i.e 1/3)?