An easy way to introduce this into your sex life is while enjoying yourself missonary style grab his ass and slowly move your hands towards his crack, once there pull his cheeks apart slightly and brush one of your hands along his crack. He may hesitate but say something sexy like "You are making me so hot, fuck me deep". This will spur him on. Now brush a finger over his asshole. Do it a couple of times. This should start the process. Good luck.