There is a way where by you dont ask at all but make it a suggestion as in "You need me as..Or you will enjoy me as, or You must" As a dominant to plead or ask... Your defeating the purpose..It is your job and responsibility to take the lead in all things...So take the lead here.....Remember that the training is one of the most rewarding aspects of being a Dom...Make him aware of the advantages of him being YOUR sub..You will be the provider of untold delights to him or his worst nightmares...