Bend his cock back and masturbate him from behind. In this position he is very vulnerable as both his ass and balls are easily accessible for you. The more you pull his cock back the more he will push his ass out towards you to ease the strain. This position also makes it impossible for him to pump his penis against your hand so you are in full control of what stimuli he receives.
Oh! Interesting….
Yes, really quite hot this one. But also fun to do more casually as you sit next to him and watch TV or something. Just leaving his cock hanging down sometimes, maybe as you get yourself off… Also, plug his bottom, it’s the perfect situation!