While that's his choice... if you've never had anything in your ass before, not even a finger, then, quite frankly, his request is a bit short sighted as your sense of submission will be everything he wants if you are not distracted by the pain/discomfort I suspect you will experience.
Whereas, fingers, slim buttplugs, up to thick buttplugs, that you can insert yourself at your own pace in advance of accepting his cock, will acclimate you to the stretching.
I have used my girl's ass many times, starting her off on buttplugs and culminating with "live entry".
It has only been recently, now that she has become accustomed to it, that I forbade her taking anything into herself prior to our meeting... and I must admit it was much more difficult for her than I expected.
Bottom line, I don't know of anything you can do to prepare if you're not allowed to physically prepare.