Nothing can be sexier than sliding your finger(s) inside of your partner's ass.  It's tight and warm and very smooth and can be very pleasurable for your partner.  Men have the added bonus of a p-spot a few inches in.  Using your fingers is one of the best ways to massage your partner's prostate.  This can be done with a partner or solo on yourself.

Take some lube and rub it around his asshole.  Use the other hand to massage his taint and balls while you encourage him to stroke his cock.  If you are doing this solo try using a mirror for some visual stimulation.  The anus is a muscle so massage it slowly to get it relaxed for penetration.  Warming up the area can feel amazing with all of those little nerve endings there.  When you think he's ready slowly begin working in a well-lubed finger to the knuckle, crooking it towards his balls.  (His prostate is approximately 2 inches inside and is described at as chestnut sized bump).  Some guys like a gentle thrusting motion and others prefer intense constant pressure.  So I’d play around with the different stimulation to see what feels the hottest for him!