Hi, You could possibly , nonchalantly touch his ass while you are giving him a blow job. My wife did this for the first time last week. I got so hard. I had wanted her to do this for so long bit I was afraid to ask her bottom line. There is something about touch in/around my ass that gets me off so bad. It has since I can remember. Nonetheless, Maybe some are more prone to enjoy it than others. Good luck to you.
Btw - I am 100 percent straight and love my wife and have always loved sex with women.