Hi
The cheapest and best way is for the son to execute a registered gift deed in the name of the mother at the sub-registrar office.
The whole process of executing a registered gift deed at the sub-registrar office is a one time affair and will be completed in maximum 1 or 2 days.
The stamp duty and registration fees are almost the same for both registered gift (by the son in favour of the mother) and the relinquishment deed(by the son in favour of the mother).
However there will be no tax benefits in case of relinquishment deed and transfer by relinquishment deed is subjected to capital gains tax.
As per the tax laws, the term 'transfer' includes relinquishment, not gift. Hence, when you are relinquishing property for monetary consideration, it will result in capital gains for the transferor(you).
Hence a Gift deed is the cheapest and best option.
Hope this helps