See firstly you have already given him vakalatnama for.the ongoing case so no additional vakaltnama is required for case to file extra document or application in same court .
Further he may also keep this if required for other court proceeding and you are travelling and Vakaltnama shall be of his name it cannot be misused as such.
So you may give him vakaltnama if it is required by him as if he intend to misuse he is already your lawyer and has signed VP in court.