Well, I don't really know how to explain how to do it, but I will try to help anyway.
First, he has to be super, super gentle! Also, I know with me, I can't have anyone rub right over top of it. It is so hard to explain, but it kinda hurts, but is also kind of just an annoying feeling.
But, if you/he rub around it and find your "spot" you will know what to do.
My suggestion is for you to experiment on your own and see what feels good and what doesn't. Then show him what to do and what not to do. It may take some time to get it, but just relax and have fun trying. But, if it doesn't feel right or it hurts, chances are you aren't doing it right.