For sure, keep a finger up inside the asshole against the prostate and just as he's reaching orgasm you'll feel his prostate swell up and then feel the asshole cum around your finger as your jerk him off to a full strong satisfying orgasm. I bet he would have cum 3x as hard if the finger was up his asshole during the orgasm.
He should have had a finger deeply inside his asshole massaging the prostate while he reached orgasm so he could feel his asshole cum around his finger and triple his orgasmic pleasure.