i'd suggest you lie down and very seductively ask him to come with
your finger and while you do so tell him that there is something very
big (or any other word of your choice) and exited under your
underwear. and he will wonder what is it.
i think it is a
very sexy way to introduce it, as it will make him more aroused with
the introduction and the temptation of finding out and so very
smoothly he will love it.
really good luck.