Yes, stretching can helb for preperation. In any case: lubricity is important, a must so to say. Good luck. :-)

Before you even get to the stretching part, be sure you are relaxed. Tell him to service you first, kind get you all warmed up, suck you clit play with your g-spot, then make him bring you to orgasm a few times, if he really wants the backdoor, he has to do a lil work first. Then while he is say giving you oral your front and back are becoming very relaxed, so he can slip a well lubed finger or two back there and if they go in easily then your ready, its not a matter of stretching the anus to fit, its a matter of bein relaxed enough to fit something in without too much pain pressure and effort. Now that your this far, have a small vibe or even just your hand ready,as he is going to try to get it in there your natural reaction is to tighten up back there, if your rub or use a vibe on your clit then it will keep you relaxed and you should allow him to just slip his well lubed penis inside. The key here is lots of lube be relaxed, and if it hurts alot then your doing something wrong and you should try to get more advice, let me know how it works out, we make anal part of our sex practice quite a bit, only one time did we do it before she was ready, and yeah it went from the front to the backdoor by accident, it hurt really bad, but doing it the way I said we have had no problems. Very long answer but when it comes to backdoor sex the explantion shouldnt be a 1 line answer.